Insulation isn't really "needed" in most cases.
You will always come out cheaper to build yourself rather than buying prebuilt.
Plywood is best for the outside, but you could use cheaper paneling or OSB on the inside where it will stay dry.
Spacing the 2 X 4's on a 2 ft center can save a little there, and using metal instead of shingles for the roof is cheaper also
You just have to accept that new material prices are high, or scale back the plans to fit the budget