the best way is
determine how long the project will take From Start to Finish (your best quess) - include all time needed - aka actual making and preparation and travel
calculate what will be fare compensation for that time:
there is no formula or standart rate, especially at the beginning, so just pick the number that would make you satisfied.
calculate the materials and other expences
add those two numbers and add 15% for "unexpected"
you got the price...
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