What will you be using this for? Most people use them for plinking and home defense. It's a 10/22 that accepts. 9mm, that's it. It's very limited in its design and not the first carbine I'd buy if looking for one, as there are smaller and lighter models out there with equal or better reliability. The Ruger was built for budget police departments so the cops would have a 'rifle' and not have to switch ammo. $600.... If you're in that price range I'd get a SU2000 or Beretta. My 2 cents....
If it is one of the older "Camp Carbines" and it is 99%, it very well could be worth the asking price. These came in 9mm and 45ACP IIRC. Although these had "pallet wood" on them(white birch) it was finished pretty well. I'm not really sure weather they were drilled and tapped for rings or not. I know the top of the receiver did not have integral mounts like most Ruger's have nowadays.
See what you find on Gunbroker and the like. Mags could be a problem for this weapon also. You might want to give XD45 a PM. I seem to remember that he had a couple of these at one time,he might know more about them. --- SAWMAN