There will be wildly differing amounts it could be depending on many factors from average local plumber rates to removing an existing water heater first.
You've already spent $20,000 on water heaters and you're just now checking on installation costs?
Why do you need that much Hot water capacity? 260 gallons is a lot of hot water to keep heated. We use a 50 gallon household type water heater, no one ever complains about running out of hot water. .
I have 1- 240000 btu boiler on a 6 and 1. This is shared with floor heat and hot water and a fan coil unit to heat the auto bay. On the very busiest days the temp of the hot water goes down to 115 instead of 125. We have a 119 gallon storage tank. I would try to return those water heaters or worst case sell one on craigslist and use the other.