A Fantastic Fab with the cover is pretty inexpensive if you get the basic reversing model and no thermostat.
I have had one in 6 different trailers and NEVER had water inside at all and have been through some impressive storms too!
So for me it is the ability to save a lot of money and have a great fan with no chance of water entry not even melting snow!
If I needed to fit in a shorter space with a trailer I might get something lower profile but otherwise I don't understand paying that much more for a fan?
I got my fan on Amazon for $175 shipped, thought it was a good deal. I got it for $100 less than the local RV place here.