To install a concrete floor you need the following layers (starting from the bottom):
1) 100mm mechanically compacted hardcore (ideally 20mm shingle)
2) 25mm mechanically compacted blinding sand
3) 100mm Jablite (or equivalent) underfloor insulation
4) Layer of 350mu polythene membrane
5) 100mm C25 concrete
6) 50mm screed
7) Your chosen floor finish
Materials costs (approx):
1) Shingle £30 cu/m
2) Sand £30 for 0.6 cubic metres (1 tonne bag)
3) Jablite approx £9 sheet (1200 x 2400 x 100)
4) Polythene £35 for 100 sq/m roll
5) C25 concrete approx £65 - £90 cu/m
6) Screed £90 cu/m
These are net trade prices and vat + delivery costs must be considered. You may find you cannot buy at these prices
If you measure all this up you'll see you need to excavate about 400mm+ below finished floor level. You may find there is a concrete oversite already under your joists but this may be too high and possibly needs to come out?