Factory rack is pretty awesome once you get a cargo box... mine is designed to latch on to the factory cross-bars. Its basically a very simple C channel type clamp... I can install it myself in less than 3 min. Empty its light and manageable.
I was the same as you, it sounds expensive until you actually get one and start using it...
I got lucky and got one used for $100 but after using it on two long road trips, I would buy it full price again if I had to.
For the amount of space I get on top its an incredible value... and considering space inside is at a premium, it was a huge upgrade for me. Might not be the most stylish option but it works a lot better than it looks... and that also takes into account adding weight up top and aerodynamics. (It was a lot better than a flat rack in terms of both)
I have to go off memory but this was my carrier box inventory for that trip...
1 x Craftsman tool bag stuffed with tools
1 x Viair 400P Air Compressor
2 x MaxxTrac traction pads
1 x Sleeping Bag
1 x Sleeping Mat
1 x Coleman 3 Person Ground Tent
1 x Shovel
1 x Fishing Pole
1 x Laundry Bag with Dirty Clothes
...along with other small bits and bobs scattered in spaces around the edges.
Since then... I got a flat rack but if I am honest about it I do miss my cargo box at times... Its a lot difficult to load up a flat rack with loose goods like I have above all while keeping it secure from wind, rain, weather, and even theft depending on what we are talking about...
I miss the ability to just open it up and toss something in.
I still have a carrier, but I need to fabricate a new mount so I can keep using it. Flat rack has its own advantages and disadvantages, but until I get my cargo box back it feels like I end up carrying less stuff sometimes.
Flat rack has been great for pelican cases and folding tables... basically larger things that are easier to tie down. It really is a different application... think about what you want and the performance impact as well. For all those bulky soft goods you want out of the cabin, nothing beats these cargo carries in my opinion. Loading it that way also has the least amount of impact on handling and performance, at least that how I see it from my own experiences.