IggySD

I stayed at the Herdmanston when I went down there. Probably a bit more expensive than it should be but it's in a very quiet part of town (except there are tons of crickets or some other loud nocturnal animal) and has very friendly service. Probably one of the nicest places in town but don't expect 5* service as in more developed countries. You could certainly find somewhere cheaper but it has a certain charm.

Regarding the falls tour - I think a lot of the reservations are made within a few days. Guyana is (was) still a largely cash based economy so going to the office with money in hand saying you want to go tomorrow isn't that bad a plan, there are usually a few extra seats. I want to say the cost was somewhere on the order of $150-$200 but can't remember for sure.