Define 'cheap' ;-) Now, if you want to talk about 'affordable' ...
All the degree programs at Western Governors University are competency-based - independent of 'seat time' or, indeed, how/where you prepare for them. Theoretically, your only limitations are how well-prepared you are coming into the program (previous training and experience), and how quickly you can prepare yourself for the other required competencies. Tuition is based on 6-month terms (graduate level: US$3250 plus $45 library fee). That works out close to US$130 per week; textbooks could add another US$40-50 per credit hour, so budget for at least US$150-160.
With this system, you'll have a LOT of written assignments, proctored exams, and performance evaluations, so if you're highly self-motivated and disciplined, you can easily complete all the degree requirements faster than traditional brick-and-mortar - 26 weeks instead of 14, for example, and NOT being forced to sit through dull, dry, boring lectures by disinterested professors or teaching assistants ;-) Clearly, it's NOT the norm, and YOUR actual mileage will vary, but since I've been there we've had a number of students finish their bachelor's degree in four or five terms, and master's in two or three.
Personally, I had a rather slow start returning after 40-odd years since high school, but I'm on track to finish both degrees within a cumulative six terms - comfortably but full-time (as in five or six hours a day, six days a week). I couldn't do that anywhere else, and certainly not at such an affordable price.
However, like all the other variables, you have to decide which ones are relevant in your siituation. For example, if you can't put in as much time as I can, you might not realize any noticeable savings over going at it a few courses at a time at a higher tuition. Just don't get sticker shock at the price of tuition or textbooks ;-) In the long term, you'll be rewarded many orders of magnitude - and those rewards are not limited to mere financial remuneration, although that is certainly a prime motivation.