Statistical methods to determine the proportion to min. value of depandant variable

I am new to this forum, but heard alot about it from my classmates. I am basically doing an internship in a company. The company is using various channels for recruitment where it is incurring some cost and also facing varying degrees of attrition. I have last three years, month-wise data with me, involving costs and attrition cases according to the hiring channel. I have to statistically analyze the data, so as to propose the percentages of hiring from different sources so as to minimize the cost incurred and attrition faced. In other words, I have to statistically find the optimum percentages of hiring from different sources to minimize the cost and attrition rates. How should I go about it?