What's the cheapest accredited online college?

There are a couple of options. For a school that has Regional Accreditation (RA) which is the same accreditation as brick-and-mortar schools the cheapest is Western Governors University (WGU) where you can graduate in as fast as 6 months and as cheap as $3500 depending on how quickly you can move through the material.

You pay the $3500 per term no matter how many courses you take and use a competency based model that means that if you can pass the final assignment at the beginning of the course you’ll be awarded the credit.

If you’re fine with a Nationally Accredited (NA) degree that is less widely accepted for graduate school you can get a Bachelor’s degree from University of the People (UofP - not be confused with the for-profit University of Phoenix.)

The University of the People provides online education “at cost”, they subsidize things so your Bachelor’s degree is just $4,000 in exam fees and you don’t pay for any other material.

Hope this helps, good luck.