This is one of the main reasons why I switched to an outdoor haunt. It is FAR easier to find outdoor space on the cheap than indoor. I managed to rent a four-story, 100-year old mansion in the late 90s for $2K a month, and that was the BEST deal in town. Nobody else would touch us.
Think smaller. You may not like that advice, but you may have better luck 3-5 years from now if you've successfully run the event. OR putting that money into a large party tent and building under that (even with a sprinkler system) would be cheaper. In my experience nobody with a large space will accept an unproven seasonal event in a business that doesn't always have the best reputation. Trust me, I've tried, but my personal success came by going a completely different route. Haunted forests proved far more lucrative and creatively fulfilling...and a heck of a lot cheaper.
Look to places like Pirates of Emerson or the (now defunct) Rocky Point Haunted House as examples of this. Start small, build a reputation and eventually you'll be able to reach the size you want.