upgrade!
Some people build a stand to accomodate an equal sized sump to their display tank. This is way more expensive though! I think a good sump size is about 30% of your display tank, so you want to make sure the cabinet you build to have room for a 15-20 gallon sump.
A new stand or a sump? Sumps are dirt cheap, get a second hand tank or just make your own acrylic one with store bought acrylic, use some acrylic sheets and make your baffles, buy a drill bit somewhere online, some PVC pipe, bulkhead and thats about it. If you have a spare tank lying around even better even cheaper.
A new stand, well maybe it can be more expensive to get a new stand just for a sump, but again you can make your own, which would again make it dirt cheap.
If a used 50 gallon tank is not available, it will run close to 100 bucks instead of a 20 gallon which is like 40? Also, acrylic for a 20 gallon, baffles will be maximum of 15 dollars, whereas for a 50 gallon it will be about 45. so that's 145 vs 55 (without the cost of RO water). If you can get a used one, then yes it would be cheaper. Lumber for the stand will depend on prices at your hardware store, but a much bigger center area is required for a 50 gallon, than a 20 gallon.
Anyway, I was just saying that relative to a nano tank, running a 50 gallon with a 50 gallon sump may not be "dirt cheap".