We've had a few discussions on this group that, IMHO, show a need for both the need for theory as well as good practical hands on experience.
Many things that a purely theoretical appraoch might disregard as impractical or unworkble are neither. Many things that are obviously unworkable or impractical would never have been tried had the doer been a little more experience.
I think the space between the two is commonly called "engineering." There is a tendency on this group to ignore this middleground. The half-assed projects are pure fun, no hope of working, but probably a fun way to spend a saturday afternoon -- if your lucky with some spectacular pyrotechnics. The theoretical models strive for a precision that is virtually unattainable in this field. (I remember a debate about a 100th of a second and how, in the end, IMHO rediculous one hundredth of a second is when applied to something that is, at best, 1/20th of a second varibility.)
The phrase "good enough" is tossed around like a label of poor quality. In reality, almost everything we use and depend on, day in and day out, works on the basis of "good enough" and low (not no) probability of error. The trick is to mitigate the error when it happens, and insure that errors are very infrequent -- if ever happen at all.
Neither I nor my $500 robot project, is striving for a "just good enough." I am saying that with proper engineering, a home built $500 robot, built mostly from commonly available parts, is a very workable solution. One which will rival almost anything one could buy outright of a similar approach.
As for the $500, it is a round ballpark figure. Assuming that the average robot builder will have a cache of stuff that can be used. The real cost may be a bit higher if one were to itemize and price out the components, and a bit lower if they have something like a Fry's near by or a good collection of junk.
Regardless of whether or not you will build my $500 robot, it is my hope that some of my work on my project is useful to people working on thiers. I am making an effort to present real-world workable approaches to common problems a robot builder will encounter.