I would guess without concrete figured in since they gave you a quote per square foot on the floor.  We are at around $10 a foot for building and materials on our steel buildings.  That includes r-10 insulation but does not include doors as I sub those out.  Really not a fan of sliders at all and pretty much refuse to do them!

I would look for steel as it is just a mater of time before all your roof screws or nails start to back out and leak when driven into wood.