This differs from course to course. Sometimes you qualify for a scholarship because your university has an agreement with Utrecht University or HU University of Applied Sciences, sometimes a foundation sponsors specific target groups, and in other cases the course leader gives a waiver to a few outstanding students. The availability of scholarships is clearly indicated at the course pages, under "scholarships".
In case Utrecht Summer School doesn't offer scholarships we advise you to contact your home university or local institutions for financial support. You can also have a look here for more information about scholarships.