Re: how much money
what region, labour rates vary from area to area, as do materials plus disposal
around here its roughly $0.50/sq ft for a basic 8 ft ceiling house with no matching existing plaster. some guys will only do a bond coat and 1 skim, better guys will do 2 skims and you wont see a single seam.
also in some area of hte states disposal fees are based on the volume. where as here we pay by the ton though the rate varies based on what exactly your getting rid of.
im with havana, call around your local guys, ask the guys on the contractor desk at your closest lumberyard or drywall wholesaler
fire up the saw and make some dust