Rat bites, hell just about any sort of bite, can get infected very easily. Even if it doesn't look like much on the surface, you could have some nasty bacterial action going on underneath that you won't know about until its face begins to swell with pus.

Suck it up and spend 35 bucks to take it to the vet just in case. (Seriously 35 is NOTHING. I've had to pay 60-70 to just get checkups for one of my herps.