K.P.
Please don't buy them an expensive bottle of champagne. It's like throwing away money just to make a point... which they won't "get". You can spend $150 on a bottle of champagne or throw $150 in an envelop.
I always do something off of the registry for a shower. If there is nothing on the registry that I can afford, then I do a gift card to that store with a moderate bottle of wine for the shower.
For weddings we always give cash. Depending on "who" they are... the amount ranges from $150 (friend's wedding that we couldn't attend) to $300 (immediate family and we attended the wedding).