The current tier limit is 20M sP spent in a single purchase at the Millionaire Center. Since there are a lot of items priced well over this amount in the MC though, I think this achievement's a good candidate for new tiers to be added. :)

I kind of like this idea but currently there's only 7 items above 20mil in the MC and they split like this:

27mil - Thorny Rose Dagger 40mil - Double Chakrams 52mil - Gilded Item Tome 150mil - 30 Percent Discount Card 400mil - Ether 450mil - Ornamented Hook Swords 750mil - Natasha

If they added more tiers they would maybe need to add more high expense items since those are pretty limited choices (mostly weapons) with quite high sP cost jumps in places. A couple more items in the 25-75mil range would be nice.

On if the yet to come antique shop is made an extention of the MC.