Yes, caster level can be 1st, so they would cost 4,000 gp if following the formula blindly, but that would not properly reflect what you gain and thus not be a good idea. If your DM doesn't allow it, so be it, he's completely right there (and it's not the point of the formulas to be used that way, only as a guideline for the DM to gauge magic item costs; the final cost is set by the DM not the formula)!
They should cost 18,000 gp IMHO.
I'd rate it just like most enhancement bonuses, where level² (level being the factor by which 10 ft. is multiplied to get to the speed increase, 3 in this example) is figured in as a base and then multiply it with 2,000 gp simply (using Longstrider as a base instead of Expeditious Retreat, which is also done with the Boots of Striding and Springing).
So cost would be 3² x 2,000 gp = 18,000 gp.
Bye
Thanee