Best Cheap Bicycle Carrier?

I've got the Thule bike rack that straps to the hatch. It carries two bikes just perfectly. Once it is set up you can mount it in 60 seconds and unmount it in 20. I ride about 2 times per week, so it is perfect. Put it on just when I need it or leave it on if I feel lazy and don't plan on running through the automated car wash. The only downside is you can't use the rear wiper when it is mounted.

 

Since you specified "cheap", I would recommend the Allen Basic 2. I haven't used it with the Prius but it works on hatchbacks (had one on my old Matrix). I have also owned more expensive trunk mount racks that were much more difficult to use. This thing is really bare bones but is great if you just need it a few times a year.

 

I lucked into a Rhode Gear Shuttle 2 rack for $15 at the Goodwill store. A friend saw it and told be about it, so I told him to grab it. I used it on both of my Subarus, one a coupe, and the other a wagon. It fits the Prius well too, but it straps on, and you can't open the hatch with it installed. It is very stable. If its just you and your bike, just throw it in the rear hatch, even two bikes should fit. The Rhode Gear racks were around $80 last time I looked. My rack is similar to the one on the above post (Allen Basic 2).

This is the one I have (not my ad):
RHODE GEAR SUPER SHUTTLE 2 - CAR BIKE RACK

Here's a good deal on a new one:
Rhode Gear by Yakima Super Shuttle 2 Bike Rack - Save 42%

 

I have one of these Allen basic 2's (about 15 years old too!) and it works fine for carrying 2 bikes. Takes about 2 minutes to put on, and 30 secs to take off.

 

I was going to mention that too. They have made that Allen Basic design years and they last forever. As the name implies, it is indeed very "basic" but it's a proven design. If it ain't broke...

BTW, don't get lured into the Bell racks they sell at Dept. stores like Target. They look similar to the Rhode Gear Subarutoo mentioned but are a nightmare. Trust me here. I've heard the Rhode Gear one is good though. I do like the integrated straps and notches for the cross bar.