Generally, how much does invisalign cost for only the top row of teeth?

Depends: Sometimes treating only one arch is more complicated than treating both.

Too hard to say: You are not buying a commodity like a banana. Its like saying "what does a lawyer charge for a case"-- depends on the case. Same with braces/invisalign. Most of what you pay for is their time. If you have a difficult case, it costs more, easy case-- less. Without seeing you, they'll never know. Indeed, i would not trust any office that has a "one price fits all" for invisalign.

Cost: Cost varies according to complexity. Single arch treatment often more difficult, may take longer and cost more. Single arch treatment may damage your bite. Clear aligners only about 1/2 as effective as braces. Talk to an Orthodontic Specialist about your problem set and your treatment requirements.