I would start, on craigs list. You may get lucky and find a reasonable deal. You can always offer a low price and see what happens too. Most people are just trying to offload some tanks to buy bigger ones.
I would check out your local petsmart or petco, to get some ideas on price. This way you can judge if you have a good deal on your hands. ALWAYS find out when friends and family is and ask for coupons!!!
I think buying piece by piece is better than buying a kit. This gives you the abiliity to customize your tank to your wants and i believe you will get an overall better complete product. You may end up spending a few bucks more or even less but atleast your buying what you need.
Of all the sites I have seen and stores. Petmountain.com seems to have the best savings not on tanks, but on everything else. This is my opinion but compared to other spots i have purchased they seem to have great deals. I would also not be afraid to purchase a tank that may have a leak because of seals. I recently re siliconed a tank and it cost around 10 bucks for all supplies but its fairly easy and Now i am always on the hunt for a tank that i can repair and save a ton of cash. I have enough supplies to repair 10 tanks now.
So to answer your question on a price. It's really how much your willing to search, and maybe fixer up, or ask for deals at the big stores. I recently bought a 45 gallon tank with fluva 203 caninistor, heater, full lights, wood bottom stand and matching top, tubings, heater, glass top lid, for 70 bucks. I felt that was a good deal. I found this on craigslist.
GOOD LUCK. VERY EXCITING TIME FOR YOU!