it might be an old version of cura - but I much prefer it to the current version. Better layout and way easeir to use.

This is a tricky question. Particularly if you also do the modelling then you need to charge for that as well.
People often say: 'how much ? for a little bit of plastic.'
You then have to explain that it's also for your time and expertise and for the fact that it is a custom made object that usually can't be bought in a shop.

Whatever number you initially thought of charging - double it.