$20 per tree is possible on the material cost side. Labor is another thing. Here's how.
Lets use a 24" average DBH as an example here because the math is easier. The BEST bio-stimulant that I sell is called Bolster (from Sustane Corp), NPK ferts are much cheaper so I'll ignore them for the sake of the example. Let's assume you've altered zone timing & sprinkler nozzle size to correct the dry/wet spots and some rain rounds out the drought stress for you.
Bolster per 2x2.5 gal case = $205.90(APPROXIMATE)
Apply in a 2'x2' grid in injection sites4-12"deep
Mix @ 1 oz per 100 gallons:
$205.9 divided by 640 oz per case = $.32/oz
.32/oz X 100gals = $32.00 per 100 gals.
At 2 gallons per inch DBH cost = $.64/inch DBH
So a 24" DBH tree costs YOU $15.36
Now for NPK fert. Unless we try to waste money at a retail store, we should have no problem obtaining a Soluble Fertilizer for $1.00/Lb.
Adding it to the tank @ 3-5 lbs/100g = $3-$5/100.
So a 24" DBH tree costs you $2.40 (@5lbs/100)
Labor costs vary be region. Your 200gl tank will provide material for 100 cumulative inches @2gl/" or 4.2 trees per tank. 75 trees divided by 4.2 about 18 tanks for the job. Say 19 tanks as the teank will never be empty. Filling will require appropriate adjustments for what remains in the tank. Hydrant fill @ 20+gpm and the tank is filled in 10 minutes. If the sprayer isn't 100% empty & the men don't shut off during refill, then one man is filling water while the other is adding fert/bio-stim. Thats still a minimum of 3 hours spent filling. (if they never had to move the truck). Double that time so they can move about the site= 6 SKILLED MAN HOURS for fill/move.
Actual inject time for 4 trees/tank @ 2g/inch depends more on the skill of the men than anything else. Lets say that the leaf-canopy spread is 15'. Since your trying to do the best job possible for the site ON THEIR BUDGET, we'll make injections on 2' centers. Since we don't treat the center 50% of the inner drip line, we can make the following conclusion. Each 15' spread tree will require a minimum of 52 injection sites. ( I dont have time this AM to share the formula, but it's close) This yeilds 3 rows of injects beginning at 50% out from center all 2' apart out to the drip line. ( I know, we can do better, but not on this budget.)
If pressure is adjusted so that each inject site is held for 3 seconds, then we have 3x52 injects or 156 seconds on the trigger or 2.6 minutes. You own the sprayer and injector, so calibrate with a graduated bucket & stop watch. Double that time to allow the applicator time to find his next site.
5.2 minutes per tree is fair if the men have the motivation to move. If they never had to move the truck, they could do a row of 75 trees in 390 minutes or 6.5 hours. Add to that the 6 hours we gave them to fill and move= 12.5 hours. 2 men.
So we have 25 skilled man hours.
We discovered a cost/tree of $17.76 each or
75 trees = $1332.00
We're not digging ditches here so you plug in your local SKILLED labor rate, then sell the job, then crack the whip.
If labor=$55/hr x 25 = 1375.00
Materials = cost 1332x2(50%GP)2662
Or $4037.00 for the 2 day job. The sold price per tree came to $53.83 per tree. And the men have time to take lunch and scratch their butts a couple times. Any negotiation in price is a compromise of your profit. We could use a lesser biostimulant to save cost. You could just sell the NPK fert or just water alone for that matter. The aerating effects of water injecting relieves compaction (a little) & benefite the drought stressed condition to. ( add a lot of time if just watering).
We've drawn some fairly vague conclusions based on the unknown site terrain, but if a cost of $20 per tree was the only hurdle, we slammed it a while ago. No matter how bad the client wants to save money, it will cost a lot more to R&R dead trees.
But that's just my opinion. Good Luck