Welcome!!
I would also recommend the 20 gallon because this hobby is addictive and you will want a larger tank sooner than you think!
I have a 12 gallon Nano-Cube and I think that I have spent upwards of $350.00 or more.
My tank cost about $100-$150 (I'm not really sure because it was a Christmas Present).
But then I spent almost $140 on
LR and
LS.
I purchased my H2O from my
LFS because it was easier for me. (So lets guess $40 there)
My hermits and snails ran me about $20
My shrimp and my clown were $75.00
I have two Kenya Tree Corals and I think that those were $30
Plus I purchased Calcium Buffers and testing kits as well as odds and ends for about another $50.00
SO...if I add these numbers up I get between $$415 and $465.00
While this seems like a lot it was not all spent at once and was spread out over two months.
The biggest investment if
LR,
LS and the aquarium.
Hope that this information helps.
Ask Lots of Questions. This site is the best place to find answers.
Good Luck and Have Fun.