Also, what about siding? The cheapest thing I can think of is 1/2" CDX plywood. Is there something cheaper (besides what I can scavenge for free)?
Varies locally, you would have to shop around. You could use 3/8" if you are pretty sure you have no really determined raccoons and will keep it in good condition but that would not save you much over 1/2" I don't think. Really, if you want cheap, scrounging is your best option, just see whatcha can come up with.
For the shed roof, I was thinking about 1/2" plywood covered with some fiberglass corrugated panels. Is that a good/bad option?
There are likely better ways of doing it, but that would be ok (especially if you happen to already have some of the materials lying around). (Are you certain you have a source for ACTUAL FIBERGLASS panels? In most parts of the country they are hard or impossible to find anymore, having been replaced by pvc and polycarbonate plastic panels. Do not use the cheaper ones, the pvc panels, as they are very NON heat/cold/wind/impact resistant and will die an early death from sunlight. The polycarbonate ones are the way to go if you can't get true fiberglass; unfortunately they are usually a bit more expensive than metal roofing)
THe best roof depends a lot on where you live. What kind of summers and winters do you have? If your weather is mild year-round, metal roofing on purlins (i.e. bare beneath) is good and affordable and very durable. If your summers are mild but winters spend a lot of time below freezing, you may need to add insulation under the metal.
If your summers are very hot, metal roofing is better installed on plywood or OSB, and that will give you some reasonable margin of insulation against wintertime condensation too. White polycarbonate (NOT pvc!) plastic roofing could be used instead of the metal, over the plywood or osb, but is a bit more expensive.
If your summers are not super hot, shingles over plywood or OSB is a perfectly reasonable option and usually cheaper because it is usually not too hard to find shingle 'leftovers'.
I'm really trying to do this on the cheap, and my calculations already have me in the $700 range. I'm hoping to get that closer to $400 or $500. Any suggestions?
Mainly, spend some time scrounging around, see what you come up with. If it's a lot of shingles, build a shingled roof
Etcetera. You can adapt your design to what materials are available.
Good luck, have fun,
Pat