Well...you do get what you pay for.

There might be some "cheap" floorpans out there, but odds are they're thinner metal, poorly stamped and poorly shaped, and require much more work to fit properly. On the plus side, all Spitfire floor pans are basically the same '63-'80, with the only real difference being in the outboard seat mounting points (from the high-back Mk3 seats on, the seats were slightly wider).
If floors are basically all your car needs, it would be worth paying for the "good" ones for fit and for safety, which ultimately translates to greater value of the finished product.