What is the price that is fair in your opinion?
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.
I'd say this could be $20 - $30 forever or more. Sublime is $80 for each version, Synergy is $40. That's your market area.
$8 is high for mobile, $8 is CHEAP (it makes it seem like you will cut corners) on PC. Your market isn't casuals, it is hardcore users who will splash the cash for something.
I love this idea actually.