How much to install a replacement electric cooktop?

If you have an existing cooktop, the wiring is already in place. This puts you ahead of the game. Costs will vary, and they cannot be estimated without reviewing the job and seeing what is involved. An evolution of this nature could cost from a couple of hundred dollars to a couple of times that, depending on who is bidding it. In any case, that individual should be a qualified electrician because a bad job could result in property damage, serious injury or death. Best advice is to make a few calls and asking for "ball park" estimates (if anyone will give one), or have someone come over and show you the numbers. These estimates usually don't cost you anything, by the way.