There are a few RUM actions on here for sale. For that price, you wouldn't be getting a rifle, but a custom or customized action.
For $600, it is probably is fair as mentioned for parts, however to take advantage of a RUM, it needs to be longer than the factory cuts it for. By the time you do all that, you could have had a custom action. Since you want a different cartridge, may be worth it. Most RUMs do not get shot very often unless they are braked. May be dirty, but may still function just fine. Clean up the barrel and it may be a shooter. Get a clamp on brake and you will fall in love with the RUM. You'll want another barrel and brake to make it your own. Careful, you get a little RUM in ya and ya just can't stop.