How much does it cost to redo air vents in a house?

We live in a house that was built on a slab and has pebbles. Due to the pebbles, our house is always full of dust and anytime we use our heat or AC, it smells like pure dirt. I know that can't be healthy. Anyway, no one will clean the vents because it would be pointless and the pebbles would just release more dust. We were told we'd have to completely seal off the baseboard vents and install new air vents that run through the ceiling. Does anyone know roughly how much something like this would cost? We only have 6 rooms in our house that would need vents.