Polyurethane doesn't build well. It doesn't adhere well to itself. Multiple layers require sanding between layers to increase mechanical adhesion. It's difficult to put down more than 1/64th thick at a time. Plus, poly has to dry before it can cure. The thicker you build it up the longer it will take to cure and harden. Finally, poly us 50% solids, so thick layers will shrink and can crack.
Since poly is only 50% solids you would need twice as much poly vs epoxy for the same thickness. That makes the two equal in cost... because epoxy is 100% solids. Epoxy can be done in two or three thicker layers. Both poly and epoxy will cost about $3 per square ft for a 1/8th inch thickness.