This idea has been covered on Less Wrong before. I'll spend the next minute looking up some links.
EDIT: Years saved: Cryonics vs VillageReach
Against Cryonics & For Cost-Effective Charity
There's already discussion about cryonics and charitable giving in the Reddit, help me find some peace I'm dying young thread.
There is a discussion thread in Normal Cryonics about charity vs. cryonics. See in particular this comment.
Wow okay. I didn't expect to find such good arguments. I am still not adjusted to the intelligence level here. Well, different new discussion idea then.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.