Price: my PC says 25 merc coins, the help menu says 75, and when I bought a merc I ended up spending all of my ~50 coins.
What's worse is that my merc left me when I got nicked several times, to little effect, by a blade trap at an entrance. I apparently nudged the controls a little too much for my invincibility to continue.
As for how effective mercs are in general, I can't tell since I killed monsters before he could fire a shot.