I hope you got your ballpark figure for having the drive paved. You need a solid base to put both the asphalt or concrete on, and it needs to be graded to the shape you want your drive. That may or may not mean you have to remove, add, or leave the gravel as it is.
I have gravel on my drive, and checked this past week on asphalt. I was quoted $.87/sq ft for a drive 600' x 10' wide. It would be higher if it were smaller, as moving the equipment in for one drive is a fixed cost, I would assume. If the asphalt people are already in the area, that can work in your favor as well. I would seek out a local, with some contacts for references. The history of your gravel drive will tell you if you need more base, or are good to go on what you have. Mine has been stable without spring soft spots for 34 years now. I don't plan on doing anything other than taking most of the crown out that I have now. Some work on water management will be necessary, as I don't want to have such a deep ditch on the upper side as I have now. No difference that I have been aware of as far as the local assessment goes, between asphalt, concrete, or gravel. But wouldn't put it past the assessors (controlled by the state) to add something in.