OK what type of plywood are you planning on getting? I just went thru a reflooring project in my house and I went looking for luann plywood, and could not find it in either Homedepot or Lowes. They do carry something new to me--popular 1/4 inch underlayment. It looks to my eye to be better quality than the old luann stuff and almost seems to be impregnated with resin. Here is the spec sheet:
5mm plywoodNo mention of resin so I guess that is just the appearance of this material. Slightly less than 1/4 which would be 6 mm. It's $14 a sheet.
1/2 CD grade plywood was running somewhere around $25-30. You probably need around 7 sheets if you do the whole roof so your $500 is more than enough. Probably $250 will do. As for the waterproofing that material is only $56/gal and 2 gallons will more than double coat that area. If you use eternabond that stuff is pricey but $100 should cover that. I'm going the bondo route myself.