Good leather is DEFINATELY worth paying/saving for. As for ruining finishes, all holsters will wear a finish just at a different rate. Keep your holsters clean of dirt/grit and always check the finish on your pistol to minimize blueing wear. Many swear by Kydex, and it definately has its place but I have always liked leather. You may wan't to look for some of the lesser names as their price and delivery times are much better than the big names. Off the top of my head Bullard in Texas comes to mind, but certain there are many other good small makers. You can also fing good deals on ebay if your know what you are looking for.
Good leather is the only way for me. I've got holsters by Galco and Kramer. I've heard good things about Wild Bill's too, as well as DMBullard, Rafter's, Don Hume, and a number of others.
Spend the money on a quality holster and belt, and I promise you will not regret it.
Any holster can cause finish wear since there's friction when you holster/unholster.
How much wear depends on the holster material, tightness of fit, cleanliness of the holster, your care of the holster, and the quality of your gun's finish.
Hard and fast "rules" are difficult to come by. Lots of variables here (including your climate).
I've seen handguns stored in leather holsters for years that were none the worse for wear, and others stored in leather holsters for years where the holster clearly played a role in the corrosion of the finish (due to trapped moisture and the tanning chemicals used, I believe).
Starting with a quality holster stacks the deck in your favor.