My local RV dealer recommends bringing the old one in to compare and match the hinges (which is what matters).
Since I had their catalog, I was able to do the math on which one I needed. Standard replacements are $15, but they seem to only last 1-2 years. Yesterday, I picked up some Camco "unbreakable polycarbonate" vents from the dealer for $30 each which have a 5 year warranty, so we will see how much better they hold up.
In my MH, the vents had some air noise that annoyed me while driving, so I put the semi-round covers over them. I ordered those on Amazon for $17 each, but my local Menards hardware store sells them for $20. The dealer sells them for $35.