We thought about this for awhile, and I read a TON of threads on OOT bags to see what people found really useful, etc. We decided to get just a few nice things - so we're doing pashminas for the ladies (under $4.50/pash) and sombreros for the men (under $2.50 per), plus some of the flip flop note pads, mexican themed coin purses, and customized pens from OTC.
We're in the same boat as you - 35 bags - and wanted to stay around $10/bag. We tried to pick items we thought our guests would like, keeping in mind the avg age (no children coming), etc.
Good luck!!