TREE-age G4 is $465 per quart... I'm pretty sure it goes down in price when you buy in bulk. I get it through Midwest Arborist Supplies. No discount shows on the website, but call and ask... They do have early order programs with nice discounts - but those have passed for the year.
Since you are treating a large number of trees, you might look at staggering treatments. How close is the "main" infestation to you? I know you said they are in your county...but are there massive death of trees within 50 miles of you? If so, yeah...treat them all this year (actually, probably last year and 2017...). If it is just showing up and tons of trees aren't dying yet, you probably have a little time. Treat your best trees this year (or maybe the trees in half of the parks?) and then the other next year. The only reason I suggest that is to spread the cost to every other year (and this is an ongoing cost) and to spread out the time it takes to treat them. Depending on how many you are doing, you could have a couple of workers doing nothing but treating trees for 2-3 weeks. If you can make that 1-1.5 weeks per year there should be more flexibility. Final thought: DON'T treat every tree. There are probably ash trees in the parks that you wouldn't miss anyhow...if it is tucked in a grove of other trees and removal will be quick and easy, just remove it. There are likely other trees that just aren't healthy anyhow or have very poor form. Don't wast money keeping a tree that should be replaced anyhow.
Good luck!