It all depends on what you want and expect to get out of it. It is like any other firearm, only that the M14 culture is a little more niche, parts are usually rarer and/or more expensive...
You can buy a basic Rem 700 for about $500....but you can also build a 700 precision rifle costing easily 10x that.
A Norinco or Poly will run $400. How you approach your mods is entirely dependent on what you want to do with the rifle.