That said, there will be a greater cost in setting up (test kits, decorations, accessories and salt for changes). I would guess with making up new salt water every week for water changes is even more expensive that what I do to cut or increase the mineral content. In fact, a lot of serious marine aquarists buy an R.O. unit so that they can make up their water without the addition of surplus minerals in the tap water like phosphorus since that can help trigger an algae explosion. The good news there - RO units in dollars are half the price they were 20 years ago (when I got mine.) In real buying power they are even less expensive.

I don't blame you for feeding those few guppy fry off, but guppies have been used to start a nitrogen cycle in an marine tank. One can also use one of those anemone fish to cycle an aquarium like yours, but some of them are not only hardy but very territorial. (That may be a great place to do the "fish-less" cycle.) If you got one blue Chromis, that might be the only fish you could keep in your ten, because (in such close quarters) it might kill every new tank mate. :(  

There are books and columns in the magazines these days on those nano-cube aquariums. Initially there was a lot of skepticism that such small (12 gallon) aquariums could be stable marine tanks. Being pretty much square they do have nearly maximum surface.

Opinions have really changed in the last couple of years. For ideas with your conventional 10-gallon tank. Look around for what people are doing with their nano-cubes. These are freshwater planted tanks (3rd image down) and I have mentioned this site before on GL, but maybe one can get an idea of what a person can do with such "small" tanks.
http://www.well.com/user/debunix/fish/myFishroom.html