Your going to pay 3 times as much for 3,000ifb then 2,800ifb. The high alumina content in the 3,000ifb will definitely make it less pervious to attacks from fluxes but won't outlast 2,800 brick three times. Any ifb will be attacked from flux during forge welding.
When you get up to these high temp alumina refractories it's always cheaper to buy castable. I'm not sure what your trying to accomplish? If this is for walls and crown I would recommend casting from mizzou a cheap high alumina noninsulating castable. You don't need much for structural integrity. Mizzou will stand up to high temps almost impervious to any flux and is extremly abrasive resistant. Then you would back it up with a fiber insulation. For the floor I would use the same mizzou but cast it a little thicker still fiber underneath. There are comparable products from all the big refractory dealers just ask everyone knows mizzou.
I'm going to guess you already looked up refractories in the yellow pages?
Pottery stores will charge you a premium in my experience.