Another good approach is to calculate how much the board (in case of
microcontroller board) would cost if entirely produced with discrete
logic, as 74HC..., then you calculate the economy it would generate to
be done using a microcontroller (and your software), then you estimate
one year production.  Your software cost can be in between 3% - 15% of
this one year savings, with a minimum and maximum values, lets say
$1,000 and $200,000.  Sometimes you don't work by hour, but by helping
your customer doing more profit, so he shares it with you.
Wagner.