Making people pay for something that they already get for free is hard. I have never paid for a piece of software. I have donated to a few open source projects and have contributed code to several too. Most of us developers are used to getting high quality open source software for free and being willing to contribute back to it for that reason. Being asked to pay for it instead just leaves a bad taste in my mouth. If that model doesn't work for your software, fine, but that just means I'm not going to use your software.
Perhaps a better solution here would be to hand the code off to a trusted group of volunteers for clean up purposes.
I know this is going to seem like nitpicking, but most developers will never contribute to open source. Maybe most of the people you are immediately surrounded by do. But a lot of open source is written by a small subset of the population with an extremely long-tail of contributors that will never come close to giving back as much as they take.
Perhaps a better solution here would be to hand the code off to a trusted group of volunteers for clean up purposes.