I only take my cat to the vet when something is wrong, like she didn't go for years.. but she's an only cat with no outside contact and is a complete little monster at the vet. My vet is pretty cheap though, like I went for a check-up before having her teeth worked on and he only charged for the pre-anesthesia blood work, and has waived the fee for a few different things over time. A lot of vets will vary their fees I think based on situations. You shouldn't be afraid to call and ask about pricing beforehand though.
If your two cats have different appetites, maybe the one cat getting sick is nervous eating in the same area as the other. Maybe try feeding them in different rooms, so he can slow down and not eat so quickly. Also, smaller, more frequent meals may help.
My cat has thrown up liquid a few times, but it seems like it was always the day or so before a hairball. Or if she scarfs all her food in the morning she'll throw up like ten minutes later.