Phil Pfeifer owns a business refurbishing Army Surplus calculators. He has a contract to buy the calculators from government sources and could purchase up to $4,290$ a month. His bid of $$4\mathrm{.}40$ per calculator had won the contract to purchase the surplus calculators. He invested $$37,500$ in an automated engraving machine and started selling personalized calculators through a network of army surplus stores and VFW posts.

Pricing was a problem, however. First he had to consider that, on average, his resellers charged $42\%$ margins and were content to sell at his recommended retail prices as long as they received their margins. Second, he thought it cost him $$0\mathrm{.}83$ in labor and materials to engrave customized messages. For several months he sold an average of $1,050$ calculators per month at a retail selling price of $$31$ per customized calculator. His wife suggested he could watch more lacrosse if he charged higher prices and sold fewer calculators. Phil raised the price to $$45$ and saw the number of calculators sold drop to $778.$

What are the variable costs for each calculator Phil sells?