So, there is no such thing as "cheap" when dealing with turtles. Unfortunately, many people think they will be a very easy pet, when in fact that is not the case at all.
Basic ideal set-up for any semi-aquatic turtle of 4" could be:
Tank (40 gallon minimun): $40 ? at least
Heater(Non-glass/preferably with guard so turtles can't bite it :$25
Heat lamp(Ceramic heat emitter/energy efficient)$35
24" UVB Bulb(needs replacing approximately every 6-9 months:$25
External Filtration (2xs water capacity) $130
Total: $255 just to start
This does not include the food requirements(as they cannot live on just turtle pellets alone.) they are Omnivores and require variety.) They need a basking platform below the heat emitter to ensure they can dry out so they don't get bacterial infections. They have to have consistent water changes and husbandry. Turtles are VERY dirty creatures. They will clog up a filter faster than you can imagine. Not too mention, they can live for a LONG time. They get to be quite large...picture a dinner plate. Then, picture at least a 90 gallon tank for that one turtle at adulthood. I don't mean to discourage you, I just want to inform you. We now have 5 turtles of 3 different species and have weathered through many of the difficulties and condition that go along with having a turtle(or 5).:lol: Hope this helps. If you any questions in specific, please let us know.