My understanding is that, to get Packer season tickets, someone has to die. Literally. There will almost always be scalper tickets available (and I count those by ticket brokers in that category, in a sense) and because of supply & demand, they will be expensive. Priciest will be the ones with big rivalries (Chicago Bears, for example) or toward the end of the season, especially if the Pack are doing well. Good luck!
Incidentally, there are some really gorgeous state parks in Door County, a bit to the northeast of Green Bay, and another to the east: Peninsula SP, Potawatomie SP and (one of our most favorite ones), Point Beach State Forest. All have electric hookups & dump stations, and some lovely wooded campsites. The reservation website gives campsite dimensions & utilities. All are popular, and may be booked up already for Packers home game times - but it's worth looking into. (They are reservable 11 months in advance.) Probably an hour or less drive to Green Bay. If you decide on Point Beach, note that I don't recommend the (very few) water-electric sites. If you camp in that park, you'll see why. :)