I have a rose gold band with pave diamonds on the top half of the band. I think that the diamonds would look nicer against a white background. Can this be done?
Yes, but please understand that it will wear off fairly quickly, and being that the underlying color is rose rather than the yellowish white of white gold it will show more quickly and much more dramatically than with white gold.
How often depends on how hard you wear the ring. Could be a couple of weeks, could me several months. Since you are talking about just the area where the diamonds are set it could be longer.
Yes, you can do just one area, your jeweler will have to mask off the area he does not want plated, I think they use nail polish but I could be wrong there. Speak with your jeweler about this.
We charge $20, some charge less, some charge more, so no, in my opinion it is not cost prohibitive.