Salary Between $45,000 and $50,000. The average annual salary of a developmental specialist assistant was $47,000 as of 2014, according to the job site Simply Hired.hile developmental specialist assistants earned $47,000 in 2014, Simply Hired reported average salaries of $51,000 for developmental specialists. Simply Hired's survey sample could include more level III developmental specialist assistants, which would account for the small difference in salaries.