Since we are talking price here do you mind telling us what you think the fair market prices are (in general) for Model Ms? Standard, SSK, and Industrial variants assuming all are in good to very good condition. I've asked before in the "What's it Worth" thread with no help. It seems like not too long ago (within the last 2 years for sure) these were the prices:
Full size used: $40-$50
Full size NIB (from Clickykeyboard) $85-$95
SSK: $90-$170 tops on ebay
SSK NIB: $200-$300 ebay
Full size Industrial: $400-$650
SSK Industrial:
Well, regardless of my thoughts on the subject, they're worth what people will pay. I just don't understand the mindset of someone who would spend upwards of $500 on a keyboard that, while uncommon, has no differentiating qualities to justify the expense. If they have an IBM industrial computer that needs a keyboard and they want the right one for authenticity, OK, I could see that. Functionally, they are no different from a normal SSK (and depending on your perspective, a 101).
I couldn't pinpoint exact values for you...by most indications I "underpaid" for all of mine by outrageous amounts (the original so-called Kishsaver for $13 for example - I think my full size oval logo industrial worked out to 40 something), but I do know that this particular auction is just ridiculous. Perhaps with a currency exchange it isn't so bad in Euros (900 = 658, not sure if that's considered expensive in Euros or not as economies vary).
Your estimates were mostly accurate as of 2ish years ago and are typical ranges when there aren't a lot on the market. I suspect instant demand (vs the long, drawn out continual sort that is more patient to wait for a good price) has gone up due to what I perceive as increased interest in mech boards and the hobby itself.
Which is a long way of saying "idk brah".