I recently purchased a second hand Century Arms AKMS. Everything on it locks up and there is no major issues except one thing. Head Spacing. I checked out the head spacing and it failed. How much do you guys think the average cost is to repair something like this?

It's used so technically it passes as long as it doesn't close on a FIELD gauge.

I say that because I'm not sure what you tested it with, and assume you might have used a no-go gauge.

Try joining akfiles.com and sending a PM to "gunplumber".  He seems to be a well regarded AK smith.

Unless you just populate a new barrel I think you have to either set it back and drill it for an oversize pin, or
weld up the slot and re-drill the barrel pin hole.

Personally I think if I was shooting com-bloc steel cased ammo in an AK I would just shoot the thing as long
as it didn't close on a field gage.  It's Century Arms and an AK so we aren't exactly talking about a precision
made weapon.  

As already stated, need to know how you checked it.
If it's off you can probably drill it for an over sized pin and reset it.