Are these KKW schools? I can see the monthly fee of $100. That's pretty comparable to what I've seen in my area. I don't have an issue with that in a commercial school. Colored belt tests, last I checked around (many years ago) was pretty standard at $50.
I heard about many people, some I know personally, that paid quite a bit of money for a BB test. $600 isn't the highest, but is, imo, too much money for a piece of paper. Now in full disclosure, I've never paid for any test I've taken, nor do I in turn charge for any test I administer. And that is when I had a school. That's just me. If this is a KKW school we're talking about, they ask, IIRC, $85 for a first Dan certificate. So we're looking at $515 mark up. Considering that the student has paid a monthly tuition for perhaps 2-3 years in addition to colored belt tests (perhaps 5-10) I would say they've invested a substantial amount of money already to the school. To charge and additional $500+ (or more) is something I don't find reasonable. I could perhaps see matching the KKW price (or whatever org we're talking about).
Is it bilking? Well, hopefully the student was smart enough to find out up front how much things would cost. If not, then it is on them. If they're willing to pay that kind of money then so be it. Some people attach quality to a dollar amount, some do not. Some perhaps don't realize that careful shopping may provide the same level of service for less investment. And to be fair, location is part of the equation. Some places may not provide a choice. In my area there are a plethora of TKD schools (just checked the local YP which revealed over five pages of schools not counting YMCA's and community centers). Some folks may not have such options.