I would like to end my relationship with an SB but I want it to end on very good terms (or even have it be "her idea").   What are the best ways to do this?  

We're missing some info here that could affect the advice.
First, why do you want to end it? Is the magic gone? Found a better one? Going back to the family? These are all different situations that might change your approach.
Also, is this a close emotional relationship or just sex (and money) for each of you? If she is emotionally attached that's one thing. And if she is financially dependent on you that's another thing to consider.
In many cases you can be honest, as long as it's not a deep and lasting relationship. She may not be as attached as you thing. In other cases she may understand if you need to concentrate on your family.
But yes it can be tricky. Interested to hear some other thoughts.

Financial difficulties. Wifey suspects. Got STD.

I'm going through that right now. Sent her a text saying...I'm not sure I can afford you anymore. She responded with a frown and...But I need you. Yes she does. She needs my wallet.

more information is needed to provide truly practical advice.  

 
SBs are obviously individuals, with their own experiences and motivations.  It's impossible to predict how any one will react.

 
Other important questions are:  Do you have an SO?  And, is the SB aware that you do?  Or, is there someone anyone else in your life from whom there would be consequences if your SB arrangement was revealed (kids, parents, partner, employer, press, etc.)?  Is your arrangement based on discretion?  Or, are you meeting openly?

 
Understanding the nature of your arrangement, what you want, and what you believe they want, will help identify a compassionate path forward.

I can't find it now, but I know Herb has written about this in detail before -- the gist of it is to present your SB with information that will make it her decision to end it.    Hopefully Herb can post a link to that previous counsel.  

This is one reason why I am extremely reluctant to have an exclusive arrangement with a SB.  Or, to be more precise, why I don't want a SB to be exclusive with only me.  If she's relying on me for 100% of her needs, it's gonna be real awkward breaking things off with her should I want to do so.  I don't know for certain if that is part of the OP's dilemma.  Nevertheless, whenever I'm chatting up a new SB and trying to create a new arrangement I always explain that I am very poly/ENM, there will be others in my life and I imagine there will be others in hers as well.  Then I wait for her response.  If she says she only wants me, I make it clear that I'd actually prefer she had others so she did not have to rely on me for 100% of her needs.  I tell her that sometimes I might not be available or I might run into financial difficulties.  And all that is true.  But the real reason is that I don't want her to become clingy and needy, and perhaps resentful, if I need to break it off.

Sparky got the core idea.  

 
I won't repeat the full post from the past. But I will recap the 2 key concepts:  

 
1. It's always about you, not her.  Tell her you need to make some significant changes due to (financial, work, spouse, medical, whatever seems plausible) issues. It sucks, but you have to deal with the realities of life.  

 
2. Instead of saying "I need to end this (because of #1)," tell her what changes you must now make. Examples:
- I am adjusting my budget so I can ensure I can meet my core financial obligations.
- I won't have as much free time now, I can only meet 2x (or 1x) a month. Or even more drastic: 1 time every 8 weeks, etc.  

 
Then let her decide if she wants to continue seeing you less often and/or for less allowance.  That gives her (the illusion of) control over her time and attention. There is no room to argue whether she is "worth" your time and funds, You are simply defining YOUR new constraints without diminishing HER "value."  If she's ok seeing you less frequently, or she's ok with less allowance (or both) you can decide if you are ok with that.  

 
There is an argument that the only exception you might make is when she has (repeatedly) done something that violates your arrangement agreement.  In HR terms, #1 and #2 above is what you do when job performance is poor - you favor the better performing employee (aka other SB's). The latter is what you do when the employee has made serious policy violations - you council that she is not in compliance and terminate if necessary.  

 
Even then, my typical advice is to avoid a confrontation about her behavior. I had a long term SB, an aspiring chef, who started asking me to fund short trips to nearby cities so she could attend food trade shows.  I was happy to do this, as I felt an occasional $300-$400 gift in furtherance of her career goals was a worthwhile use of funds (we are supposed to be "paying it forward," right?).  Then she blew it. She asked me to fund a 3-day trip to Las Vegas for a trade conference. The problem: I scoured the web to find the conference website. There are probably 10-20 trade shows in Vegas at any given time. But for these date - nada.  Next, I decided to check her IG account to see if she was posting pics from the show, as she had done in the past. But instead of pics of her at cooking demos or attending conference sessions, she posted pics of her in a bikini in a major Strip hotel pool party. But that's not all. In those pics was a 25-ish stud-muffin beefcake due with his arms and lips wrapped around her. And finally, I checked HIS IG and saw more pics of them and pics of HIM at a conference for a different industry.   When she got back I went to see her and flat out dumped her. I told her it was just time for me to move on and I did not mention Mr. Douche Asshat.  Why? Because I refuse to teach her how to get away with bad behavior in the future.  

 
Now some of you may call a halt at this point and say "Hey Herb, why should you care? Once you give her cash it's hers and she can use it however she wants."  This is true.  But the cash is not the real issue.  I do not mind investing in a n SB's future. But I refuse to fund some Douchey Ass-Hat's access to her pussy.  

As GaGa Gambler used to tell us so many times (Miss you GaGa!):  
I will beg for pussy,  
I will pay for pussy,  
But I will not beg to pay for pussy!  

 
Life is good  

 
The Cat

… And this perfect response is why you are the elder statesman of this forum.  👍👍

My wife is suspicious is perfect.  Even if it isn’t true