It doesn't have to cost a ton, but it can depending on what you are looking for. If you simply want a functional tank to grow plants and fish...that can be quite cheap regardless of size. If you want an work of art, then you can spend a fortune.
Fluval Edge: Style wise this tank is a work of art (room centerpiece). ~$125 for a 6 gallon tank.
Tetra Aquarium Starter Kit: Style wise this tank is all function (kinda fugly). ~$25 for a 5 gallon tank.
I give you this example not to belittle Edge owners, but simply to demonstrate the aesthetic costs of the hobby vs. the functionality costs of the hobby. The two get confused a lot. I LOVE the Edge, I want one. That said, both of the above listed tanks will grow plants and hold fish. One could actually make the argument that the Tetra setup is better equipped to do so. The filtration is better (my opinion) and the lighting system is easier to upgrade for plant use (add a 10 watt CFL for $2).
Many things in the hobby are like this. Nothing wrong with either one, just the cost of form vs. function in most cases.
PS: If you can avoid buying anything with the word "aquarium" stamped on the box, you will automatically save 50%, lol.
Welcome to TPT!
Buying used, asking online on craigslist and kijiji for free old stuff or cheap stuff has gotten me everything in my sig for well under $1000 and I have which is outdated now I think. lol. Setting up a 20gal I got for free. Needed a good cleaning but came with aquaclear filter, light, the works.
If I buy a used canister, I fill it with bbq lavarock and plastic pot scrubbers. $4 to fill a canister or $100 in fancy biomedia.
CFL bulb, $2, aquarium plant bulb, $10
48" hardware store bulb, 2/$6. 48" Aquarium bulb, $40 each.
It goes on and on but if you are frugal, take your time, you can get lots of deals. I've bought boxes of misc stuff online for like $20 and ended up with like 4 aquaclears, air pumps, heaters, decorations, all kinds of stuff.
Most of my stuff though has been posting ad's just saying, anyone have any old aquarium stuff sitting in their basement they don't want and want to give it a good home where someone will use it and enjoy and I get all kinds of replies.
Anyways, welcome, enjoy the hobby and if you need anything, post in correct sub-forum and someone may be able to offer help or a cheap alternative.