You're completely missing the point. He's not proposing to write a compiler as a way to make money; he's trying to get his living expenses covered so he can write this compiler that he wants to write, and which he believes will be useful to others.
I think you might be selling yourself a bit low. The market value of someone at your level of skill is at least $8,000 per month, and I don't think you should be going down by 62.5% (as opposed to 20-30) just because it's a fun, open-source project. If it's genuinely commercially useful, you should be shooting for market salary.
I would set the same price but make the promise 2 months, if it were me.