From what i've seen on the forums, anywhere between $2000 for a cheap conversion to $4000 for top quality. Could be wrong though.
Who says paying more gets you better quality. It's either a shotty job or a good job. No matter the price.
I'm converting my VS to manual at the moment. cost so far is around $700 minus 200 for the auto which I sold. But that has included 2 gearboxes 1 of which will be resold after I rebuild it. Used the best of the 2 to make 1. Only thing wrong with the second was rusty pits in the cluster gear due to it having had a bath in a flood. Will replace that and go over the rest then sell it or do a convert for someone else. Auto loom is easy but you have to do it right.
Dont forget to use the right oil for the geabox when done. Many people do that wrong and wonder why the gearbox is hard to change gears and even popping out of gear. They don't like gear oil at all.