If you're looking to stay on a budget, I got this set from amazon and they have been pretty decent.
A couple negatives:
They do make a howling noise sometimes at 40-45 mph but it's usually not too loud
The end caps in pictures 4 and 5 of the amazon listing did not snap-fit into place properly so I lost one to the wind and took the other three off. I think they would stay in place with some sort of glue that is malleable (i.e. not like super glue) and would allow them to remain removable
Here's the camelcamelcamel listing so you can see the price fluctuations. The lowest price they've been is about $76. I bought them for $100 because I figured they wouldn't be going back down a while and I was right - they've been going up since my purchase.
More expensive options are:
The Skyline System from Yakima