If you have to ask, then it's too expensive! Just kidding. Now are you talking about installing a full sized IMAX? or just the smaller newer types out there? Just the projection system?
You could probably get used equipment for a few hundred thousand.
Someone just did it here in Arizona a year or so ago...the Peterson home @ Silverleaf. The IMAX portion was completed for the bargain price of $2,000,000.00. A very modest sum considering the whole house was about $155,000,000.
http://www.arizonafoothillsmagazine.com/features/features/1544-peterson.html
You will find IMAX-Like claims on many products in the $10k - $100k range...however, your not going to have true IMAX without spending upwards of 7-figures.
But why would you even want to. You wouldn't even be able to pop in a Blu-Ray...as you have to have specifically formatted content to play...and unless you have a distributorship/licensing agreement with the studio's...you won;t be able to obtain IMAX content.