You are right, it depends on crop (yield, yield price), region, year, and farmer strategy. In Russia pesticides cost is normally between 8 and 28% of yield unit cost. 13% - for spring wheat (fast crop, low yield, low protection), about 19% - for soybean, in the range from 8 to 28% - for sugar beet. Overall, average price of pesticides on local market is fast-growing (17% for last 3 years).
I have an review article on pesticide uses of pesticides in agricultural areas of Turkey, as follows:Conference Paper USES OF PLANT PRODUCTION PRODUCTS IN AGRICULTURE
It depends on infections by pest, if there is infection or disease development by pest, then only pesticide used, sometimes not use, if cultivation is good and well, no infection ,then what is the requirement of use pesticide, then if infection, then pesticide use which depend on type of pest, area of farming, kinds of cultivated crops, duration etc.