It will be a lot cheaper, in my experience. I have set up everything from 3 gallon picos to 150 gallon tanks. 20 gallons will be plenty of room, I'm sure, for a pair of clownfish and possibly a 3rd fish. Water quality will be easier to manage than an 8 gallon, but I would say its about the same if you do it right ( I started with a 3 gallon for my first saltwater tank).
The easy route is to buy a HOB filter (Aquaclear 50 or 70 should do, roughly $50), get a Hydor Theo heater ( 100 watt is enough, about $20), the new maxi jet power head pro (400 pro, rated at 110gph powerhead, and 500gph circulation pump, $20), now head over to Marco Rocks for 15lbs of dry nano rocks ($42) and 20lbs of dry sand ($30), go to a LFS and get 5 pounds of live rock (~$30 assuming its around $6/lbs).
That's less than $200. I didn't add salt mix or anything because you can probably find a LFS that can sell you saltwater, I wouldn't bother making my own for a tank this size. For lights I would probably go with
Aquarium Lighting: Coralife Aqualight Quad Tube Compact Fluorescent Strip Lights
That would bring you up to around $300. Close to the price of one 8 gallon Biocube. You can also cut the HOB filter buy making your own over flow chamber if you are handy with silicone
and cut the cost by $50. 
I go to TCU, $40,000 a year and 2 saltwater tanks in my room. I am pretty good with conserving money without cutting corners and risking a major disaster.
