Is noise is a concern?
If not, then a on-wheel is probably the cheapest smart trainer… Tacx Flow ( $400ish a friend had it, and said it sounds like a train coming at you)
If you want silence you can get the 4iiii Fliiiight ($500ish). Its decent, absolutely quiet (you would need to shell more $$ for something as quiet) , accurate power, BUT apparently better for people who max FTP will be maybe 250 to 300.
IMO the road bike/power meter is the way to go.