I've used this site before to buy parking ahead of time in Detroit for a discounted price:

http://www.parkwhiz.com/ford-field-parking/

To me, the $12.99 Park-Rite deal probably is the best. I've used that garage for Tigers game before and all worked out well (although it actually was cheaper for those). You might find parking cheaper or a few blocks closer, but to me it was worth it to know exactly where I was going and not have to worry about it.