I think it would be great idea to implement such achievement to reduce wow-inflation significantly. At the moment the only way to 'exclude' some money from internal turnover is Sand Drake (you are spending money to vendor mats, so they are 'wasted' and not coming back). According to current AH prices for new weapons and FL loot, average cash player owns is like 200-300k - and it will grow up while doing FL dailies. So I am sure 'Millionaire'-achievement would lower prices twice and let them grow not that fast.