how much would it cost to replace the optical drive

I have a 48 month old mac pro. I use the optical drive quite a bit for listening or downloading music from my cd's. There is now a loud mechanical noise coming from it , and it is now very choosy about what cd's it wants to play. It is also getting very difficult to eject without having to restart it several times as well as pressing subsequent buttons.

I have read several discussions on these pages and have gone through several elimmination processes . Including cleaning discs.

Now i fear that there is no option but to replace the optical drive.

How much am looking at spending and although its not a perfect remedy , is it cheaper to buy an external drive as much as that would pain me.

Thank you.

Ray.