first - always round up, just saves time.
so $50.

second, the easiast thing to use to calculate a standard fee is the build volume.
Unless you are using a special - expensive - filament then a standard cost per cm3 is the simplest way to go.

So if you're happy with $50 for what you've printed - work out the volume of the parts and divide $50 by the number of cubic centimetres. That's your base charge.

Volume works as it incorporates both time taken to print and filament amount used.

And count yourself lucky you've got someone willing to pay for something :-)