I don't really think that there's a general concept of fair price. I'll come back to the idea that the typical user for your product has probably spent several thousands of dollars on their system. Because the context is productivity, the value of your product to them is not tied to being cheap. It is tied to making the person more productive. For the target user, how much is staying in flow worth over the course of a year?

It isn't a consumer app. It's not going to sell a million copies over two years. Maybe at $4.00 it sells at thousand copies. That means supporting a 1000 users for $4000. At $40.00 maybe it sells 80 copies. $3200 to support 80 users isn't great, but those 80 users made a hard decision based on ROI not low cost. It means that they are more likely to be happy with what it does and live with what it doesn't do. Because all it needs to do is what it says it will do for those users. And $40 is not a lot of money for a productivity boost. $400 isn't either...but that might be stretching it.

Anyway, at $40.00-$400.00 it makes economic sense to talk to every customer, and for what they paid they will not find it objectionable. Anyway, this is something with very low network effects. One user buying it doesn't add value for the next person buying it. Raw numbers of users don't really matter. At best, there's word of mouth benefit from users. That means the product and experience need to be really really good.