For the tower only (no blind), about $300.

I've started using the steel "Unistrut" channel pieces you can buy from any big hardware store for my blind towers. Not only is it plenty strong, but it's super easy to use, because it comes with plenty of convenient pre-cut slots over the full length, so you have plenty of holes available to bolt pieces together. You simply cut the pieces to the length you want with a hacksaw or 'Sawz-All" and bolt together through the existing slots. It literally takes me about 1 hour to completely build a sturdy tower from this stuff by myself, and I can put it together on-site. I use the 1-5/8 X 1-5/8 X 10' pieces for the 4 legs and main cross members at top and bottom, and the 1-5/8 X 7/8 for the ladder and cross-brace pieces. They come already plated or painted, so they last a long time without rusting, but I always camo paint the platform anyway.