Inexpensive parking at Cologne airport

I live in Mannheim and usually fly out of Frankfurt Hahn, so I know where to find the 3 Euro per day parking there (will gladly share if anyone's interested) and was wondering if anyone knew of anything similar in Cologne. Train tickets are going to cost about 59 Euros each roundtrip (more than my flight!) and I'm hoping that I can drive and park for cheaper. If not, I'll gladly take the train, but I'm all about cutting costs wherever possible. Or maybe someone knows about some DB hidden deal that I'm not aware of?? I appreciate any help!

When are you travelling? You could possibly use my garage (gratis), I don't live far from the airport and I think it's €3.20 on the tram/S-Bahn.