I've used gift card trades for VIP in the past, which worked well. I was wondering if some merchants are more valuable than others, e.g., does a $40 Target card give you more VIP days vs. a $40 Home Depot card?

You've asked a good question. TER used to use Paygarden for GC payments:
http://www.paygarden.com/
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I have never been able to find an answer to your q on-line but I always ASSUMED that a Walmart or Target card was easier for them to re-sell, hence, more valuable to YOU to trade-in than, say, an Omaha Steaks GC.
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Anybody know for sure?

I would love to get the real answer also ?

Yes, depending on which store's card you use, you get a different amount of TER VIP days.  

Go to My TER, then to the last drop-down on the right side, Buy VIP.
Scroll to the bottom of the list to Trade Any Gift Card. It gives a list of how many days certain gift cards will get you. See the bottom of this post, I cut and pasted some of the info.

Personally, when I found out a Home Depot $40 gift card gets me 42 days, it's been my go-to. Some merchants' $40 cards might get you fewer days, such as Best Buy is $40 for 34 days. There's a way to see the whole chart of which card and how many days it gets, but I'd have to search more to find it again.

 

 

 
Trade Any Gift Card for VIP Membership
You can now pay for VIP membership for The Erotic Review by trading a popular gift card. Instant access, 100% anonymous, no credit card required.

We accept gift card balances between $40 and $500.

Example trades:
$40 Best Buy → 34 days of access
$75 Home Depot → 72 days of access
$200 Lowe's → 229 days of access

I also see the same STATIC example list of BB, HD, and Lowe's. But are those numbers current? (They haven't changed in a while.) The DROP DOWN LIST would be current but you to have a card already in-hand to see its value (and I assume that the trade-in value comes instantaneously via links to and from the monetizing company, paygarden).  
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The question, or at least MY question is: Which card should I BUY today for the best trade-in value TODAY?  I suspect the trade-in values vary with paygarden's supply and demand. Christmas time? They want more toy store cards so the trade-in value goes up. And so on.  
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I have not found a way to get the trade-in values IN ADVANCE from a list without having an actual card (with value on it) in-hand.

But the sample list is static and old. You need an actual GC number to get its value from the drop-down.

You've made some good points about the limitations of the system and the chance one takes without having a gift card readily available in order to know its value. However, as I stated, I use Home Depot's $40 card that gets me 42 days, so I know that part of the list is up to date. There may or may not be a better deal with other cards, I do not know, but some $40 cards only get 30 days.

The example isn't helpful because I wanted to know if a $40 card is always 34 no matter the merchant, or if some merchant cards get you more days. So if you got 42 days for $40 HomeDepot, that implies that maybe some other merchants gets you even more days per $40.

So if you can remember how to see the whole conversion chart, please do share. Or if there's some variability based on supply/demand as impposter theorizes, that would be nice to confirm.

Does anyone know if TER still uses Paygarden to facilitate GC payments for VIP?
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If so, does anyone want to volunteer to contact paygarden and ask them? (I would NOT say "GC for TER." Rather, I would ASSUME that -- or maybe even ask them if -- Paygarden only cares about the GC and whether they can resell it and that payout amounts are the same for all vendors and maybe even the same for individuals.)
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If you want more info, google "companies that monetize gift cards" or similar. Here's a current link (but they don't include paygarden; maybe paygarden is only for corporate clients these days?).  
http://www.toptenreviews.com/best-gift-card-exchange-sites
Best gift card exchange sites 2022: Gift card exchange for cash.
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If you contact any of those non-paygarden companies, their answers might be general for all monetizers.  
Once again, different strategies can apply. Q: "I got a bunch of GCs for my birthday and I want to see which of you monetizer companies gives the best rate for each one. How much for a $50 BB? How much for a $50 Omaha Steaks?" A-1: "We give the best price for BB of all the companies. Competitor-S pays more for Omaha Steaks." A-2: "Don't worry about it, we all pay the same rates for the same cards. BB, you 87%; Omaha Steaks, you get 63%."  And so on.  
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Let us know what you find out!