You will most likely get a lot of different prices on equipment and livestock based on experience. However, just know that 15G doesn't give you much room for error in the way of water conditions. In the
SW world, there are a lot of factors that determine a healthy tank. I once had a 30G that was challenging for me to maintain. Since I switched over to my 90G last year it is so much easier.
Startup costs on a 15G are pretty minimal depending if you go
FO,
FOWLR, or REEF. A reef tank will set you back more than a
FOWLR, so you would want to decide first what your plans are. If you want to keep corals, plan on getting a good lighting system, filtration, and a good skimmer. Things to consider:
Substrate
LR
Skimmer
Heater
Powerheads
Sump (optional but gives you more water volume).
Lighting - controversial subject you will most likely get many opinions.
I have seen a Nano setup complete at a
lfs for around $160.00, but I don't think it came with a skimmer.
Some other people will chime in that have more experience with Nano's than I do that might be able to get you a package deal rather than separate pieces.
HTH
Mike