Congratulations on being able to 'see the light at the end of the tunnel'. "Cheap flights" are getting harder and harder to come by, but many folks suggest using Kayak.com to search all sources for the usual airlines. If you have patience, you may also compare prices for different days of the week or for specials to other seasonally active airports, like Puerto Vallarta, if you might like to stop there and take a later flight or bus to Guadalajara. Monterrey could be an option, with economical Mexican airlines flying to GDL. That said, no matter how hard you try, Chicago-GDL may be as economical as you can find. If available, Mexicana offers the best comfort and service includes drinks and meals.