Is it expensive to fix a misfiring cylinder on a VW jetta?

It could simply be a dirty spark plug, pull it and clean the accumulated carbon off the firing end, re-gap and replace. See if that fixes the problem. You'll probably have to do them all to find the cyclinder that is misfiring. If that fails, buy a new set of spark plug wires and replace. Finally, check the distributer and cap for carbon tracking which can steal the spark from an adjacent plug wire. As far as cost goes, even if you did all of thses things; it would likely be under $100.00.