Look into the
Saris line of bike racks. They mount via straps and metal clips to most sedans and many vans and SUVs. I've used mine on my '92 Toyota Corolla and it fits well -- holds 2 bikes (there are other versions that hold 1 or 3).
Saris Bike Rack Fit Guide:
http://www.sarisproducts.com/fit/fitguide.asp
The Prius is listed in the fit guide. Check it out and see if anything looks promising.