In general, you get hired as an hourly employee for summer internships instead of a salaried full-timer. Hence, salaries will vary from company to company depending on their valuation of intern's work so getting a bead on what's "average" will be quite difficult.

Once place to look is glassdoor.com. See if the companies you are applying to have listings there and what people have said about them, and any average salaries. For instance, search for Intern in the jobs category and you should see listings showing average salaries for posted companies, if people provided the info.