11 years and going strong IF they've kept up treatment.
I had treated at one property from 2008 to 2012. New owner didn't treat. Got back out this year with current owner and they are in decent shape...maybe 20% dead. If those had gone untreated the first 5 years they would have been toast - nine lives through that. There are almost no untreated ash left, so I think the pressure has been much less with lower big population. I am very confident that an appropriate treatment program will keep EAB at bay as long as it is followed. I was initially very doubtful...but have seen otherwise. Early treatment is the key.
To the OP:
If they are just using Tree-age alone at the recommended rate of 5ml/inch, that is a little high, but not bad. If they are using 10ml per inch (which is probably appropriate for just a few years when the initial wave is hitting the area really hard) then $13-14 is probably close to market price. I have done soil treatment (imidacloprid) every year and Tree-age plus soil applied imidacloprid every other year. I was a little over that for the combined using 10ml per inch...but have come below that now using 5ml per inch.
If you don't see any dieback, you can successfully treat with soil applied imidacloprid using .4 oz per inch of the 2F formulation. I have some going on 11 years that still look good...I was recommending stronger treatment when we started, but they didn't want to spend the $$, so chose imidacloprid and it has worked well.