I have some limited experience with caddies, mostly at high end public access courses. My impression is one pays a single share ($20-$25) to the forecaddy. Ask the rest of the group what they want to pitch in and everyone gives one player the money to tip the caddy.
If you went out as a single, you might goose the amount a bit but don't feel compelled to give him/her $80-$100 just because you are a single. The club is supposed to put together groups and if they are sending out singles with a forecaddy, that is an issue the caddy needs to address with the club.
When I have a regular caddy, my tip is more in the $40-$60 range, depending on the experience and assistance they give me. Since I don't play that many top-rated courses that offer caddy services, I try to err on the high side, figuring and extra $10 or $20 isn't going to hurt me in the long run.
Hopefully some other Oobers have experience with caddies and you can get a consensus for the right amount.