I would say the module is worth about 65% of that price at most. I have a pet-peeve about the programmer being overpriced, even at half your suggested price, but I suppose they are collector's bait these days and can often be found at higher prices than the MKS-50's and Alpha Junos. Both of those prices need to come down, as there is no real price increase justification in the difference between Alpha Juno and MKS-50.
The rack has a somewhat better Midi spec, more user memories and more chord memories, but sounds exactly the same as the keyboard versions and the rack lacks the performance parameters, such as brilliance, modulation rate and envelope time that are useful for quick tweaking.
The only advantage I found having a PG-300 over using a sysex programmer, besides the obvious immediacy of sliders you can also get from something like a Behringer BCR2000, is the PG300 has a "Manual" button that sends the entire set of slider positions at once, making it dual as an older style WYSIWYG interface.
"Lots of people are nostalgic for analogue. I suspect they're people who never had to work with it." - Brian Eno