Steel prices have at least doubled in the last year or so but that does sound expensive for only 3mtrs, I would expect to pay about that for 7.5mtr length. Steel is also in short supply and I think some suppliers have their pirate hats on.!

Like Neale says deal with a local supplier rather than through the internet and you'll get it cheaper.

3mm is too thin for the mainframe but will be ok for a base frame, I wouldn't go less than 4mm for the mainframe. Why 90mm.? That's not a common size so you'll be paying more for that.!