Since the majority of the board is in Utah, and Utah real estate prices are skyrocketing, I thought I'd ask here as there might be a good chance someone here might have found a good source.

I'm negotiating an option contract for a client who owns 120 acres of undeveloped land in the St. Louis area. It's surrounded by industrial and commercial developments. We have a buyer who wants to lock up an option to buy the real estate in the next 30 months. The St. Louis county real estate market is starting to act like the Utah market a few years ago with land and home prices starting to rise significantly. If we lock in a sale price now, but don't close for 30 months, my clients would lose any increase in value. We're stating the sale price in $ per square foot, and they have 5.3 million square feet of land we're putting under the option contract. If land prices go up by a dollar a square foot, it's a big chunk of change to them ($5,300,000).

I thought about using standard inflation indexes like CPE or CPI, but in a booming real estate market, real estate prices rise much more quickly than general inflation. Is there some sort of index or guide for real estate prices that anyone is aware of? Bonus points if someone knows of an index that is divided into regions, cities, or areas like the CPI.