If cost is a big concern, do go with a rock saddle. I started with that and it worked fine. Not great, but fine. No side dees, no cushy leg straps, no quick-adjust buckles, but it got me up and down safely.
One of two things will happen- you'll think you don't like tree climbing (hopefully not a result of using a sub-par saddle!) then you won't be bummed because you're only out 40 bucks for the harness you got, orrrrrr you'll like climbing a lot, hence you'll climb a lot, then feel the discomfort of the rock harness you'll quickly say, "hey, I gotta get me a nicer saddle. I better get on the phone with new tribe!"
It can be done, you won't die. You just might not like it all that much.
love
nick