You did not mention how many turtles you have: this would influence how much (minimum) space you'd require.
Based on personal experience, creating an indoor setup is more expensive that building an outdoor pen in the long run. Space is *never* adequate, lighting and heating add up, electricity adds up too, maintaining humidity is a challenge, cleaning is a pain, and the vet bills that might arise from screwing up on basic husbandry can be exorbitant.
The 4'x 8' indoor 'infirmary' I had initially cost us as much to put together as the 25'x 30' outdoor enclosure we built, and that didn't include operating costs.