How much should I give?

I think it depends on the relationship you have with that person. For my FH cousin's wedding last weekend we gave $200 (not like we could really spare it but what can you do.) My FMIL said that was good, but if we really couldn't hack it to give them $100 or $150. But she also said 'just remember they'll be coming to your wedding soon.'

I know they say to cover your plate...but I'm not going to change the amount of money i give to a friend/relative just because they get married somewhere cheaper...

We give the same to everyone, regardless of where the wedding is...

I agree. I do not give 'less money' because the reception is held in a less expensive place then someone else's wedding. I normally give $200-$300 depending on my relationship with the couple and my financial situation at the time. If they had it in a backyard with burgers I would still give the same amount.
I don't like the cover your plate idea because a) no one really knows how much a wedding costs and b) not everyone can afford that. Give what you can.