I'm doing soap favors (I posted them, the celtic dragon soap favors). I wanted to do something useful for guests, and we are also making a donation to the wounded warrior project. Some people do only the charity favors, and just give out a bookmark or letter on the tables saying they've made a donation to a certain charity on the guests behalf. That is something that can be inexpensive and meaningful, since it's up to you how much you donate for each guest.
As far as the soap favors go, I can kind of do a breakdown of cost for you, but I didn't buy all of it from the store. I found a HUGE lot of soap and candle making supplies on craigslist for $100 (but it was like, 4 big containers of soap and candle making supplies, I would say probably about $800 retail worth of stuff). However, if you can't find anything on craigslist, you can go to www.wholesalesuppliesplus.com
and get the clear melt and pour soap base in a 24lb brick for $41. To make 120 soaps that are 4oz each, you need like 24lbs of soap (that's how much I calculated I think). I think the breakdown for costs is around $0.75 cents/soap after adding up what I spent on the packaging supplies, plus what you'd spend on soap base and scents.
I hope that helps!