Ballpark numbers - $1-3/SF to remove old slab depending on access issues - cheapest with wide basement door, highest if need to hand-carry broken up concrete out or use a conveyor through a window. About $3-6/SF for new slab depending on local concrete and labor costs and on whether they can run truck chute in through window or door right to the pour, have to buggy it in, or have to pump it in. Vapor barrier underneath about $0.50/SF extra.
You would definitely need to confirm the existing slab and foundation depth up front for bidding purposes, because you do NOT want to be digging down to the bottom of the foundation wall with a stone foundation, because if the bottom kicks in on you the entire wall can come down. You also have to check the underlying soil conditions - to be sure you will not have to excavate deeper to replace substandard base material, or perhaps install a sump pump drainage system so the radon mitigation system does not pick up water. I have done jobs where the slab was going to or below the stone foundation wall base, and was done in narrow strips not over about 3-4 feet wide at a time - tearing out and excavating in alternating strips, replacing it with waxed cardboard or visqueen over cardboard bond breaker/spacer at the adjacent existing slab, then moving on and removing and replacing the rest of the old strips after the new concrete has gained strength - that can easily add half again to double your concrete removal and pouring cost, and of course REALLY complicates doing the reinforcing and the radon system - lots of couplings.
A radon system installed before the new slab is poured probably about $500-700 range, because certainly cheaper than havingn to install through existing concrete.
One other thought - I don't know why you are replacing the slab, but if basement is 6' deep and you think foundation goes 18" past that, why are you not considering lowering the slab to get at least a legal height ceiling - minimum 6-1/2 or 7 feet in most jurisdictions ? If you do that, you should probably get a structural engineer in to assess the condition of the stone wall and just how deep you can safely go, and whether a perimeter beam or thickened slab would be a good idea.
Of course, these are ballpark numbers - you need to get bids on a specific scope of work to know for sure.
