It really depends on the file. Some may be thicker, thinner and require support, which directly affect the time it takes to print it, and thusly the cost. You may get wildly different quotes depending on who you go through and what quality you expect. If you buy one already premade, you may luck out and get one that fits. But custom work is always going to be more expensive.
If you can, see if you can download one on thingiverse and send it to an online printing service to get a rough estimate.