With a little.. well... quite a bit of work, you can make a nice enclosure for less than $50. Our indoor enclosure we are currently using is in a corner of our room. Lined the floor with linoleum, put duct tape around edges, and built a small wooden box out of scrap wood we have... only need 2 sides if you put it in a corner.... Get some lamps, use a bowl or dish of some sort for water, go outside and get some moss and old logs.
Outdoor is better and can be done just as cheaply.. you need some chicken wire and a shovel... dig a 4x8 hole, 2ft deep.. line bottom and side with flat rocks to keep turtles from digging out... chicken wire about 2ft high above ground to keep them in.. overhang the top to keep them in..
Both of those can be done for less than $50, easily. It just takes more time for the outdoor enclosure... We built one similar to this (are in the process of it, actually), and we have about 30 man-hours on it so far.. we expect to put about 5-10 man-hours in on it before it is finished.