You can´t buy macs from the flea market or at walmart in germany. (At least not the flea markets I´ve been to. An iMac G4 (which won´t run MorphOS but is comparable to a SAM460) will be sold at about 350EUR on eBay (if you are lucky), but you will have to wait longer to find it there than to just order a SAM from Vesalia.
Why compare to an iMac? Get Mac mini or PowerMac. After all maybe you have to pay 100EUR, 200EUR, 300EUR, maybe even 400EUR but it will be much less than SAM. And even better if you dont like it you can put it sale again. Try that with SAM or any other rather exotic hardware.
Real problem with SAM (any model) is that only few hundreds or (at best) thousands will be made until it is out of production. Burden of supporting each variation with limited product runs is too great. There is reason why every Mac model is not and wont be supported.
Macs are getting more popular, so it is much easier to get an Intel machine than one of the classic models.
I dont follow your logic.
No Mac (any condition) that was built after 2000 will retail below 100EUR here.
So Macs are bad because they are so expensive. Better buy SAM instead because it costs even more? Makes sense.
(Btw eMacs should be much much cheaper but personally I consider CRT is unacceptable.)