I bought a valley hitch from Uhaul for $140 and installed it myself. I bought a 4 bike rack at a local sporting goods store for $100. I only have 2 bikes, but the 4 bike rack was only $20 bucks more and that way there can be some space in between.
I had a rack that you can attach with straps, but it did not work well, came loose and put a small ding in my tail gate :angryfire:
The trailer hitch was easy to install and only took about 30 minutes. Uhaul could have done it for $20. For me it was worth getting the hitch.