There is nothing in CS that requires your physical presence in a lecture hall to learn.
In my search thus far, I've found zero schools that offer a traditional undergrad CS program online. And please, before we get into the 'you can learn most of undergrad level CS topics online' - I would like the structure and support from an online, instructor led course that offers student assistance/office-hours. This is something I'd pay for, but because I'm working full time, I can't take classes during the workday.
Generally, 2-way interaction is done via message board, but most (maybe all?) of the instructors I've had so far keep specific office hours and available are for web chat or telephone calls during those hours.
Any of the prereqs that you're missing could potentially be taken as a transient at another institution (be it locally, or online). This is how I've handled the few classes I was missing.
Based on the rates listed on the page linked above, it should be around $25k for the whole thing (based on needing 60 credits of upper-division classes, since you already have a Bachelor's), even as a non-resident.
Also, there is an element of selection bias. Those who are likely to apply to an MS in Computer Science self-select according to their perception of the requirements of those programs. This enables programs to capture those without a Bachelor's degree in Computer Science but who still have the level of motivation, acumen, and interest necessary to succeed in an MS program in Computer Science.
So long as they know the fundamentals of programming (through, say, the equivalent a Data Structures class) I think a mathematician could take a go at it without much issue. An EE probably wouldn't have too much trouble either.