The are 3 different ways across 2 unique modes of transportation to help you travel from CGN to Köln. If you're only considering the price, you should check out the train out of Rösrath towards Köln, as it is the cheapest to Köln. It's common to find tickets for as low as $24. Train from CGN to Köln, it will get you there soon enough!
There is usually no need to book these sort of trips from CGN to Köln in advance. Rideshares should be available on demand all hours of the day.