I have signed two year leases. I have a $20 per month discount for a two year lease. Most rents will continue to go down, not up, over the next two years.
The second year, assuming they stay, you will not have any turnover costs. You save big.
I sometimes give renewal incentives anyway.
If you can put a clause in that says discounts will be recoved if tenant does not stay two years, even better.
