First of all, what kind of application equipment are you using? Dormant Oil is not that expensive and when applied with a JD-9 in proper environmental conditions, it works great on all types of scale. If you're looking for residual, try Merit granular spread evenly around the base of each plant to the drip line. Do this when new growth begins to appear. Check the label on liquid Merit for soil drench if that's your preference. Imidocloprid gives season long control on most insects when applied properly.
Don't take this the wrong way. But, if you can't afford to do what it takes to get the job done effectively and still make money, you're either not charging enough, you're not equipped for the task at hand or you're making promises to the customer you can't keep.
"The Poor Fish" circa 1930's: The Poor Fish wouldn't have been caught if he'd known enough to keep his fool mouth shut.