The best suggestion I can make is go for the largest tank you can possibly afford the first time. It will save you a LOT of money in the long run. You won't be going through the 20,40,60 .. every few months. I had 3 turtles to start with and when I realized how much it was going to cost to properly house them, I gave one to my daughter so that they could all have proper care and housing.
I paid about $150 for my 75 gallon tank. I also shopped online for the filter, lights, heater etc. The second largest cost to upgrade is your filter. Many here suggest the Filstar XP3 or the Fluval 404. Both of which run between $90-100 online, in pet stores you'll pay near double that.
I personally love visi-therm stealth heaters. They come with a protective covering already and you can get them online for about the same as you can the cheap over the side heaters you'd find in a pet store. They currently are on special at
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=12060&Ntt=stealth&Ntk=All&Ntx=mode+matchallpartial&Np=1&N=2004&Nty=1
I know this is getting long.. sorry. An alternative would be to buy a plastic/rubbermaid/stock tank, they are a lot less expensive than a glass aquarium. There are so many options and variables when it comes to your turtles home.. My best advice is... research.. research.. research.. and buy once so you don't have to keep spending money on upgrades.