18min is rather average in a noncompetitive scene. You might place but if there's any actual runners It'd be closer to 16-17 for placements.
I don't mean that as any kinda flame or calling you slow, just saying

as far as prices and stuff that all depends on location and race and what not. You'd have to figure out what's happening near you and look into it from there.
also if you're running 12min 2mile, you aren't necessarily at 18min by 3miles. and a true 5k is like 3.1miles so it's ever further, you're probably closer to 19 than 18.
If you do end up doing a race though, post results and stuff