Could they? Most complete bicycle shops probably could without a doubt, mechanically speaking. They might not be exactly familiar with the mounting but they could figure it out.
Would they, is another question. I've encountered resistance from LBSs toward MABs, particularly gasoline motors. Some do embrace electric drives.
You'd have to ask. They might cite some liability issues or some such thing.
If it were me seeking a shop as you are, I'd look for a motorcycle repair shop to do the work or a backyard mechanic.
Costs? Hmmmm, what ever you agree to. It's hard to say depending on what kind of bike and motor you have and the degree of difficulty on the install. One disadvantage of taking it to someone who hasn't done it before is they'll take more time figuring it out.
All that said generally speaking I probably wouldn't pay more than $60...thinking 3hr x $20hr. I could also see some asking a hundred.