I'm a CT Liscenced Arborist working for LESCO. I call on & supply product to the Metro NY & CT's biggest Tree Co.'s. Been working with Urban Tree care for only the last 13 years. Turf Major & all. In this area, most guys won't send out a 2 man crew for less than $2000.00 per day unless the company is of the low-ball nature. 75 trees is a long day of injecting if the trees are of size. Some things to consider:
1.) Tissue anaylsis for nutrients will determine what grade of fertilizer & how much. We sell a 30-10-7 100% Slow N from UF that would never cause a problem for the trees when fall applied. Davey Tree Co.patented this material. Nitrogen is the most likely element to be deficient. Soluble N sources could be suspect in the fall though as you seem to be concerned. Merit is a frequent addition for insect protection the following spring/summer. Spring Merit injects must be done really early to be effective. Merit will dive up the charge per day to at least $4000.00, but will save the client a fortune in the next growing season if borers have to be sprayed for, etc. Takes pressure of your schedule too.
2.) What volume of water per DBH inch will you use. In the past, the larger spray rigs (800+ gals) were popular, so 5 gallons water per inch was the norm. Today we see an increase in the popularity of 200-300 gal rigs. The water volume is decreasing to 2-3 gals per inch DBH. How big is your rig?
3.) Can water be had on site. Pond, stream or lake draw with a pump? Watch the pick-up hose strainer during fill. Check Ph of water as some pesticides are tank Ph sensitive if you ever contract to do pest sprays here. Or maybe a Fire Hydrant permit? A typical 3/4" - 1" hose bib will require hours to fill the tank if the job uses volume. Don't have applicators waiting for tanks to fill unless the job is paying for it. If they must go home to fill, this job will be hard to make money on.
4.) What is the soil moisture like at various depths? The lawn sprinklers may be providing all the water needs unless its been very dry where you are. Check down to 12+" to be sure. I've seen trees drowned by well intentioned turf managers. Site drainage, soil type, & tree species all help to determine the need for supplemental water.
5.) Is the lawn being fertilized? And are the trees feeder roots competing with the turf for nutrients or picking up a gross surplus of applied nutrients? Leaf analysis results again will help to determine or qualify the need for which elements.
6.) Cost of purchased materials for the fertilizing will have a minimal effect on the price charged. Labor is the biggest factor, so carefully use the experiences gained in time usage on similar jobs to guide you. Time yourself or your men on similarly sized trees if you've never done this before. Even injecting water into turf that has been measured and turf-paint marked to resemble the treatment area of the average size tree on the site is a useful exercise. It may prevent you from over or under charging the job. Don't ever charge less $/Hr than you currently expect to make doing similar work. Tree care is considered of a more technical nature in this area & insurance costs are higher. Experienced tree people also command a more substantial salary than LCO applicators around here. Due to liscencing requirements, tree care is a little less competitive so rates are higher. Is your area the same?
Good Luck, Steve