Pick your display. If the components you intend to keep are of a large diameter, you'll need a sump with a large footprint. A sump is only a remote container of water that you would keep things in you don't want to show in the display. For example, if you wanted a thousand gallon tank, and wanted this skimmer-
I'd say the 75 isn't going to cut it. If you wanted a 180 gallon, you would have many options for skimmers that would fit in a partitioned off 75.
The cost can vary greatly. It all depends on components you decide on. For example, lighting for a 180 could cost you 300 total, or 3000 total.....both will do a good job at lighting your tank, but one had a better name and perhaps more bells and whistles than the other.