The salesperson with a briefcase is a big expense. The average cost of an industrial sales call went up 50% from 1979 to 1983, according to recent reports from McGraw-Hill Research's Laboratory of Advertising Performance. In 21 industrial sectors surveyed, the average company spent $205.40 per call in 1983. Producers of electronics and related products were the biggest spenders (with the exception of miscellaneous services) at $274.90 per call, while the penurious wholesalers of durable goods got away with only $71.10 per call.It took a lot more gab to make a sale, too. Most company reps make 5.5 face-to-face presentations before making a sale, up from 4.3 in 1979. Salespeople in food-related industries, though, averaged only two calls before unloading goods -- proving, we suppose, that the way to a buyer's heart is through his stomach.