Underpaid, yes; But some see favorable shift coming.

States that top dollars will always go to the most visible players, but media people are becoming more valuable to agencies. Results of an `Advertising Age'/Beta Research Corp. survey; Media buyers and planners claim that they are underpaid; Salaries are higher where the account managers and the creatives work; Some executives say media salaries are starting to shift; How paychecks are handled; Senior-level salaries; Mid-level salaries; Entry-level salaries.