First, how much space do you have? Aquariums take up more space that you would think. The tank can't be flush up against the wall, due to all the stuff that hangs off the back.
The next thing you shoudl consider is what fish will you keep. Some fish get big and therefore need a big tank. Despite what people say, a fish won't grow only as big as the tank allows them too. I don't know how many times I've seen 10" Oscars in 20gallon tanks. Damn thing can barely turn around.
By small to medium I'm guessing you want something in the 55 gallon or less area. A 55 gallon tank is 4 feet long I believe. Believe it or not, 55gallon tanks are actually pretty cheap. It's pretty much the standard sized aquarium.
How much you spend depends entirely on tank size and what fish you want to keep. obviously, Goldfish will be cheaper than Lionfish. Not just in the cost of the fish, but in what it needs to properly care for it.
Maintenance also depends on what fish and tank size. A bigger tank doesn't have to be cleaned as often as a smaller tank. Fish type matters because some are eating and pooping machines.
It's hard to say what it would cost, but you could get a 55 gallon tank and stand for roughly $300 new. Make sure to check the used classifieds in the newspaper. There is nothing wrong with a second hand tank. They usually come with extra stuff such as filters, decorations, etc.. at a good price.
Make sure to look the tank over. Always check it for cracks. Obvously, you aren't gonna be happy if it empties itself on the floor. If it's an acrylic tank, look for scratches. If you find some, talk down the price as it can't be fixed. If a glass tank is scratched, it can be fixed.
Some fish to look at:
Convict Cichlids. Tough little bastards. With proper care, they actually look far better than whats in the pic. Some are pink, some have a blue tint to them. I had them for awhile, awesome fish.
African peacock Cichlids Awesome looking fish, they can be blue like this one, or yellow with bluie stripes and so on. Easy to take care of. The females are a brown color.
Blue Acara, cool fish. I had them sometime ago. Easy to take care of and very easy to breed. You'll need a bit bigger tank for these guys (45-55gallons) they get to be about 8-9 inches maximum.
Electric Blue Hap. I never kept them myself, a few friends did. They all told me they were easy to care for. The bright blue depends on how well you take care fo them. Good water and food and they get to this awesome blue color. There is also a Electric Yellow Hap that have a yellow or orande color.
Some bigger guys:
Green Terror. Yes, they are absolute Terrors. If you want one, you'll need at least a 55 gallon aquarium for one. They will eat anything they can get in their mouths. So, if you want to keep it with something, it'll have to be of equal size. Which means a really big tank. I was actually afriad of mine! It bit me numerous times! There is also a Red Terror, it gets just a bit bigger than the Green Terror. But looks just as cool.
Then there are these guys, I wouldn't recommend them to you, they can be a bit hard to take care of. Add in that they can be quite expensive, you probably won't consider them. They also require at minimum, a 55gallon tank just for 3 or 4 full grown fish.
Discus, as you can clearly see, they are beautiful. Often referred to as the King Of the Aquarium. Great fish if you have the patience to try and keep them. They come in just about every color and pattern you could think of. Some people are so anal retentive about them that they keep them in bare tanks. No sand, plants decorations, nothing. just fish and water! Some will even buy super expensive Reverse Osmosis units for better water. Some people even store water in barrels and age it to get all the chemicals out and to reduce it's hardness!
I myself kept them in a planted tank. makes them really stand out if you choose the right plants.
If you want any more info, just ask. I could go one forever, but you are better off browsing here:
http://www.aquariacentral.com/species/
Soon as you know what fish you'd like to keep, you'll know what size tank and extras to get. You'll be able to get a better idea of what it will cost.
I myself would like to go for an Arroawana if I could afford it, but I don't have room for a 150gallon tank! LOL