FWIW, as a developer if I'm willing to break out my credit card to pay $5/year for a tool, I am every bit as willing to pay $20. I would wager you'd get far more than just a 1/4 of your users if you jumped up that price.
The key question here is what percent of the potential purchasers already have a Chrome wallet (or whatever it's called now) and just have to click "buy".
Impulse purchases by these users would be more likely to drop off with a price increase.