Skip the expensive primer, protect your skin and fight oil with a mattifying daily sunscreen. Many formulas act as a matte base, preventing the production of excess oil and laying a smooth base for your favorite summertime foundation. For example, Neutrogena has some great oil-free, mattifying options (I especially like their Dry-Touch line) and Coola has several nice mineral formulas.
If you find the formulas to be too heavy, or notice an unpleasant white or gray cast, feel free to mix sunscreen with your daily moisturizer. This will help to make the moisturizer less greasy and makes application much easier.