It is probably going to cost you roughly the same whichever way you go with it, so as dan stated, you need to decide what you want to do with the system. I overhauled my A-14 a few years ago, and it was in the ballpark of $6-700. You can certainly replace the mask with other surplus or new equipment, but should expect to spend $1-200 for a new high quality mask. I have found that some of the military masks can get painful on the bridge of your nose after wearing for a few hours, so that coupled with the A-14 sucking down O2 may drive you away from that route if you aren't searching for your double lennie.
Chris