What do you mean with selling the source code? You should be very specific about things like that. Source code is governed by something called licenses and copyright.
Put in other words, there might be two or more scenarios.
1. You have a donation enabled project with pay what you want and there is a bonus tier for people that donated whatever threshold you set, like the 2 $ for example. There you make available the sourcecode for people to compile the game for personal use or to bask in the glory of your coding prowess.
2. You license the source code for commercial use. Basically people could buy license your source code and create their own project of it and make money from it.
Scenario 2 is more suited for parts of a game. Like mentioned above, similar to those assets.
I believe what you are actually looking for is a way to give incentives to pay for your games. Or pay more. Those bonus tiers are an easy configurable method to do so and the bonus content is usually something like wallpapers, artwork, soundtracks and so on. If your audience might be interested in the source code, why not.