I've bought e30 parts cars for less than $500 that were drivable and the rust was there but fixable. I've also bought cars with great bodies and a bad engine for between 750 and 1000.
The problem here in my mind is people get the clever idea to buy a $600 car and make 2 - 3k off it by slapping some putty and paint over it and making it passable to drive; because it's a BMW they know they can do it. Then the rust comes back a year later worse than ever and the unknowing owner has to scrap the thing because the rust is so extensive. (I've seen this personally about 3 times) and it seems to be the GTA area especially.
My Price Estimations for an e30 (just an opinion so relax sparky) lol
Parts cars < $1000
Decent car that has non rusted jack points or spring perches = 1000 - 2000
Car that has superficial rust, otherwise clean and lightly modified = 2001 - 3000
Car that has never been modified, no rust at all = 3001 - 4500
Never seen winter, less than 150,000k original still shining paint etc, no accidents, 100% grade A beef. = 4501 and up.
Honestly unless it was an e30 M3 that was never in snow I wouldn't pay more than 4500 for an e30 regardless of condition, unless it was completely restored to original, and I love the e30.
The e36 in my mind is worse and less valuable. #1 there are more e36 variants than e30 variants (way way more) which means they have less value right off the bat. #2 the interiors love falling apart. e30's are better made, and can be taken apart and fixed over and over easily.
Beyond that I can't comment on the values of the e36. But they (earlier models) should now be lower than a good e30. Just because of the sheer number available in the market.