And their rule of thumb is that you aren't worth the sale. Bespoke software is not a bulk commodity. They have a free product. If you're going to be a cheapskate, use it.
The notion of charging by byte, as if software were content farming, shows that you are obviously not a professional.
My rule of thumb for pricing is that if someplace /makes you ask/ they aren't going to sell at a price you'd find worthwhile.