Okay, as I understand it;
- Your husband's family openly shares the monetary values of all gifts among themselves. They will go through a great deal of effort to figure out what the value of a gift is, down to caring whether or not it was on sale.
- If gifts aren't equal, they throw a hissy fit.
- They're going to hassle you for not spending money you can't afford to visit a wedding in an expensive location.
- If you send $20, you're going to get hassled for being cheap, and giving less than you had given to previous weddings.
- If you sent your normal generous $200 you will get hassled for not having sent the previous wedding $200, in spite of the fact that you spent a lot of money attending their wedding and couldn't afford the gift.
- If you do sent the $200, you send a precedent whereby you give expensive gifts if you can't make the wedding, and cheap ones if you can, thereby making your money more valuable to the family than your presence.
Frankly, it sounds like making $20 your standard wedding gift for the family is a wonderful idea. If you're going to be in trouble no matter which you do, go for the option that isn't going to set you back.
If they freak out and give you trouble, explain that you realized that your previous generous amount isn't always possible, depending on travel and other financial circumstances, so you decided to scale back to something you'd always be able to afford, and can therefore be completely fair.
What a pack of brats!