What is the cheapest way to get from Düsseldorf Weeze Airport (NRN) to Nijmegen (NL)

What is the cheapest way to get from Nijmegen Central Station to Düsseldorf Weeze Airport and, after a trip of several days, back from the same airport to Nijmegen Central Station?

I want to minimise the cost, but the journey shouldn't take more than 2 hours and I need to be certain that I arrive at the airport on time. This means hitchhiking is not an option.

I have found a shuttle bus but it costs €18 for a single and €30 for a two-way ticket. I hope to find something cheaper.