As a general rule I believe that you should at least cover the costs of what they are spending. However, one needs to remember that the traditional offering of a present (or money) is intended to help the couple get on their way to their shared life, I know it might be different for couples who already live together and have most things, but to me the principle still stands.
Your are there to help them celebrate their ocassion but I'm sorry I don't think that you having to buy your own drinks counts, I know we all have a budget and we have to take all spending into account by I'd like to think that they invited you because you are somehow close and they genuinely want to share with you and you genuinely care for them, in that case I would make an effort depending on the relationship and regardless of whether they are spending $40.00 on your meals.
I don't want to sound harsh or offend you, I just think we can't just say well if they only spend $20.00 on me and I also need to buy my own drinks, why should I give them anymore....
I completely disagree to be honest (but that's ok, always good to have differing opinions).
I think the fact that they aren't providing drinks is very relevant. You'll already have to spend more than you would normally because you are buying your own alcohol, that comes off the present money if you ask me. I normally give $200 for a wishing well from me and my partner (for close friends anyway, workmates, relatives etc might only get $100). And this probably doesn't cover per head prices, as most formal reception prices are approx $120 per head these days. But i think giving $200 is plenty as a gift.
But anyway, if I was invited to the wedding you mentioned, if they were close friends i would probably give them $100 but if they weren't, I would consider putting in less, say $50 as you mentioned. Also, is it an anonymous wishing well, or will you put the money inside the card?
I personally don't think you can have it both ways, ie, doing your wedding cheaply, but still expecting the big presents. In all honestly, I'm surprised they're even having a wishing well. To me it just seems a little scabby to be having an extra cheap reception but then asking for cash of guests.