There really is no magic number. There are lots of different styles of haunts , building methods, and props.
Here is an exercise for you. Get some graph paper (or do this on a computer) and draw a square 80 X 75. Each block = 1'. Now draw yourself a sample maze. Don't worry, you won't be tied to it. You can now count 1 wall every 4' and that will give you an approximate build cost. Now add 20%.
Now give some thought to how you want to decorate it and what you want to do for props. Go on line and find them. Draft a budget for props.
Don't forget costumes. You can find those online too.
Now estimate your costs for insurance, security, electricity, telephone, website, advertising and you will begin to get an idea for what this is going to cost. Keep in mind that your estimate at this point is probably too low, but it should give you a rough idea of what you are looking at.
This will give you a much more realistic idea than any number thrown out on this board based on your question without any more information. It will also give you a little more insight into just what this process entails.
Have fun and good luck.
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House of Chaos Haunted Attraction