I have two large tanks, a 92 gal corner tank, and a 125 gal. long. I highly reccomend a larger system for stability and ease of maintenence. I spent about $3000 on both....give or take. (and god only knows how much since then). I'm a bit of a stickler for buying the best equipment (iwaki pumps, euroreef skimmers, maxijet powerheads, fiji liverock, hundreds of pounds of aragalive sand, MH/PC combo lighting fixtures, etc.) If the things I just listed dont sound familiar to you, then you should probably do alot more research. There are plenty of products out there that perform very well, you dont have to buy the "best," but dont buy the cheapest....there's usually a reason its the cheapest.
You dont have to build a large tank to build a good one. Find out what the best equipment is, then build you system based on what you can afford.
It really all depends on what you want your system to do. If you want to create a very self sustaining, natural environment then you cant skimp on good lights, live rock, live sand or a VERY good skimmer. If you want a decorative fish only tank, then your investment requirements go down, but so does the quality of life for your tank inhabitants.
The key is to do your research, pay attention to the what the people on this board say, and assess you monetary abilities. The two worst things you can do are 1) create a system you cant afford to run, or 2) build a cheap system that doesnt keep fish happy and healthy.
I started my 125 gal thinking I knew what I was doing. I have since learned alot about the value of sacrificing aesthetics for function. The tank I have now, is very different from the tank I started with, and I owe 99% of that to this board.