An outdoor haunted trail is the cheapest way to go in my opinion but you still have to do your homework and make sure the trail is free of hazards like roots, rocks low branches etc. I was always striving to go indoors someday but now I really love the creativity I can go wild with outside that I could never do in a building. To save money I started building props out of sticks, lol! Weather is always an issue as well as electrical depending on your needs.
Good luck and don't bite off more than you can chew. It will be very hard to make a profit the first year especially if it rains. You may have a chance if you keep your costs very low and start off small, adding to it the next year.
There will be many different opinions on this I'm sure.
A few examples.
This prop was almost free except for the time to build it.
Very cheap. Wind damaged trees and the time to carve them.
Almost free except for nails. This is slab lumber from a lumber yard that would be burned if we didn't take it.
This one cost some money and a ton of work but makes a big impression on guests. 40 foot telephone pole's aren't cheap installed properly.