First, I will say, you should not be hesitant to involve a broker and pay a fee. It will be much more expensive for you if you try to negotiate with a national tenant on your own without understanding the local market. They will walk over you...
Retail rents are a lot tougher to track then, say, residential rents. Generally with residential rents, what you see is what you get. There's too many variables in a retail rent to look at a data print out of base rents ONLY and be able to tell what "market" is.
Where is the building located and who is the tenant? How big is the building? If you can get more granular with the information, maybe we can help a little more.
As a SUPER basic example, lets say the building is 5,000 ft.
Current rents: $19.28, NNN ($96,400/yr)
Requested rent: $17.20, NNN ($86,000/yr)
Income Difference: $10,400/yr or $104,000 lease life value.(Assuming flat rent for the ten years as you described)
Value difference @ 6%CAP : $173,333.33
Scenario 1: You give the Tenant the reduction and lose 10k per year in potential rents which equates to $173,333.33 in sale value if you were to sell. This deal costs you zero dollars out of pocket and you continue with the great Tenant.
Scenario 2: You decline to give them the rent reduction and they sign a long term lease at your asking rent. This is the best case scenario.
Scenario 3: You decline to give the reduction and they leave. You relet the space using a broker (which you need for this) at $20/foot, NNN. Here are some costs you may incur:
1. Vacancy $7,166.67/month
Commission : $44,000 ( Assumes a 10 year deal, no rental increases, commission is 6% years 1-5 and 3% years 6-10)
Tenant Improvement Allowance: $15 - $20 PSF or $75,000 to $100,000.
"Total Cost" of deal excluding vacancy: up to $144,000.
Net Value on sale from doing the new deal(Value added - cost): $29,333.33
Total additional rent from doing the new deal(Total Potential Rents - Total requested Rents X 5,000ft X 10 years): $140,000 over the life of lease.
Net revenue of Lease life if you do not sell(Total revenue - Cost): -$4,000.
This example is extremely basic and assumes a TON as I don't have sufficient information. You can start to see though that there is no right answer. Your situation/desire for the future is going to weigh heavily into what you need to do.