and if she calls or writes, to simply respond that you have decided to not meet with her again.  No reason should be offered as to why even if she insists on trying to get one out of you.

 
If you want to be proactive, a short and simple note to her saying that you have decided for now to not see her for reasons of your own, and leave it at that.   End by wishing her the best and stating that you have thoroughly enjoyed your time together.

Or even a firm "weekly arrangement" but I suppose if I did any kind of bullshit excuse will work.

Since I never make any firm commitments I don't need to "break up" with an SB anymore than I have to break up with a hooker. I simply quit calling and they usually get the hint.

I would suggest one or two last meetings where you tell her that you are looking to end it (as others have said, no reason needed, but you have a built in excuse of the wife or you could go with "I just need to save more money for a while").  To me, it sounds like someone you respect and want to not do her too wrong, so it's most important, imo, to give her financial notice, thus telling her in advance of the last meeting or two that it's time for her to start looking.  If she gets pissed and doesn't want to see you that last time or two, problem solved anyway.  It should be obvious, but at this point, p2p is way to go for the last couple meets.  

Like GaG, I never get into any regular arrangement because I like to see a different SB every week if I can afford it. Sometimes I can only afford to see one a month, and I definitely do not want to see the same one every month. If I get tired of them and want to take a break, I just tell them my finance is very tight due to some necessary expenses which is true sometimes. I just had a new roof installed and had a major repair done on my car. It works like a charm when you tell them there won't be any sugar for quite a while.

Goodbye money. Goodbye girl.

I will let her know that money is getting tight next time I see her and that she should have a backup plan as I may only see her on P2P basis and not be able to keep up with a monthly arrangement that takes care of her rent.  
I am not planning on renewing my premium on SA as I already have enough on my plate that I am still talking to and still some old P2P SBs that will see me in a heartbeat, plus I miss some of my providers, lol.
But thanks all, as now I have a bag full of excuses as well as reasons not to use an excuse

Saw her about 13 months every 3 weeks. She was about 90 minutes away. Her  
 complaining about life finally got to me tho the sex was great. I just told her I couldn't get out of town as I was having someone move in with me. She was cool about it. Haven't heard from her in about 3 months an she texts me she'll be in my town an if d like to get a drink. I happened to be in her town at the moment. Told her things didn't work out with the girl moving in an asked if she wanted to get a drink next time I'm in town. She said yes. Guess there could be more great sex on the way.