Just give what you can. At my own wedding, the very last thing on my mind was how much money people were going to give me. The most important thing for me was that they were able to attend, and have a good time. I don't care if they gave $0, that wasn't the point of the wedding.
I cannot afford to throw $500 at a wedding gift, unless it's my husband's siblings, or maybe one day when my close nieces or nephews get married, assuming we finally make more money by then. My husband has 65 cousins -- we've been to probably 25 weddings over the past 10 years.... $500 a pop, I'd be broker than I already am, ha. We give as much as we are comfortably able to and that's that, I've never had anyone treat me differently afterwards. My FIL can and does give $500 wedding checks, but he also makes about $200k more per year than we do, and all of his children are grown and live on their own, so... Everyone is different. But I wouldn't turn down a wedding invitation because I couldn't send them more than half my mortgage payment for a gift...