I'm in a 1999 Volvo station wagon. For $100.00 I bought cross rails and an X-Cargo 20 cubic ft cargo box to mount on top.
This has been a great addition for me. I keep all my clothing and bedding in the cargo box because those things are fairly light and relatively easy to replace if someone breaks into the box and steals anything.
I bought a sturdy collapsible step stool at Walmart that Bob recommended on one of his YouTube videos and I'm glad I did.
My experience so far is that the effect on my gas mileage is minimal. In my view, what I gain in storage far outweighs what I lose in mpg.
I already have a tow bar mounted and I have looked at rear mount cargo carriers as an additional option. The ones that are hinged and can fold up to the rear of the vehicle are appealing to me. But I wouldn't really need one unless I started traveling much more. Also, the rear cargo carriers obviously effect entry from the back of my vehicle, where the car top carrier does not.
I found that the Thule and Yakima brands are more expensive than I am able to consider. They are typically three to four times higher in price than what I bought. The X-Cargo brand has been around for a good while and I found several used ones available. I shopped using online classified sites like Craig's List, Letgo, Offer Up, and 5 Miles.
If I can answer any questions or offer suggestions just contact me.
Drive and Thrive!
Les