To get from a working design to ready to sale, that means all the legal certifications and tests to back them up, design for manufacturing, non recurring costs custom components (PCBs, assembly, boxes), lawyers for retail agreements etc I would budget at least 10/15k€
That is assuming a standard product without expensive features (such as wireless or mains connection, where that budget would be probably spent on testing alone) and for general consumer
For automotive you can at least multiply by 5, for medical probably by 20, for Aerospace by 100
Now there are ways to skirt some regulations (selling as kit) or you could even avoid testing for CE as it is a self certification, however in case of a non compliance you would be liable for damages (IANAL)