What is "very high"? Much of the reason it takes so long to payback is labor, permitting costs, etc. If you reduce or remove those costs payback is in the order of 5 years at 10 cents/kwh.

People will point out that it's silly to factor out labor and permitting but those costs drop radically on grid scale 10MW+ systems where as they make up nearly 50% of the cost on a household scale system.