it's not common but happens and is usually the result of compromised oil. long change intervals, low oil from leaks/consumption, strenuous use (racing, heavy frequent towing), raised engine temps (which compromise the oil), ignored oil leaks, etc.
I would buy a set of used heads, or one if you're confident the one one is okay and this was just an outlier. Inspect the valves on the "good" head and make sure there isn't excessive carbon build up.
used heads are often cheaper than rebuilding existing heads and are readily available.
probably looking at $2,000 for a set of used heads, resurfacing, installing. at that point it's not a terrible idea to consider another engine if the original one experienced stress like compromised oil, overheating, etc- those things aren't good for the rod bearings either so you dont' want to go $2,000 into head replacement/repair only to have lower end issues later on down the road.
i would base the decisions on why the valves are problematic in the first place and the history/condition of the existing engine.