It all depends as it varies from person to person. When I bought a 10 gallon for a FREE turtle (I found it at work), I had to buy necessary things. I needed food, basking light, basking area, tank, water conditioner, gravel, heater, ect. Spent about $150 or so and was still missing a few necessary things.
I ended going to a 65 Gallon long tank and spent about $200 for the tank, hood/light, stand, under gravel filter plus tubing, one decor, and a few plastic plants. I then had to buy a bigger heater, API master test kit, a few more plants (3 live, a couple silk and plastic), other various chemicals, buckets, vacuum, more gravel, filter, cleaning supplies (to clean the glass), and a few other things here and there. I also bought a few fish as well to hang around with the turtle. (2 koi, 1 cichlid). I just bought a new light as well to give a better lighting for my tank. (went from low light to High). Not counting the fish, and everything for my 65 gallon setup, I say I spent about $800 from what I can think of off the top of my head at the moment.
With some things, you can get away cheaply. With others, you get what you pay for. I just bought stuff I think will last a long time coming, or have good reviews. I also bought everything new, except for the tank setup I paid about $200 initially. Everything else is new. You can also save money here and there by buying off eBay or various other websites out there.