Well there is Simoleons and then there is SimCash.
From my experience playing you earn Simoleons on quite a regular basis while SimCash is the harder to earn currency within the game. SimCash can be purchased with real money while Simoleons can be purchased with SimCash.
As for unlocking new buildings for careers, in my game at the Market Square the price of buildings go up each time you unlock a new career building, before unlocking the Fashion Studio each building showed it cost 175 Simoleons to unlock, and after unlocking the building the prices of the other buildings went up but they're still being sold with Simoleons.
Perhaps you're getting the two different currencies confused with one another?