>>They are difficult to keep as pets indoors for several reasons.
Full-spectrum lighting isn't necessarily expensive--it's relative to your budget. 
Build the enclosure out of plywood. Give the turtles an area of moist substrate in which to burrow, and ensure part of that area is covered--such that the turtles can wedge themselves into that space. I built an enclosure like this for about $50 US.
I've kept my two box turtles both outdoors and indoors for extended periods, in several states across the US over the past 22 years or so. They can be more tolerant than many people are led to believe, but it's still better to try to meet their "most favorable" conditions as best you can.
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Chris McMartin
www.mcmartinville.com
I'm Not a Herpetologist, but I Play One on the Internet