My recommendation: ask for as much as you consider yourself a fair price and worth your time. And you should also enjoy the process of making the sling. Your trading partner seemed more than happy with it as he/she even raised the price
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If you would go at it from a business perspective: French minimal wage of about 10 Euro/hour*4 hours (3 hours work + shopping for material + going to the post office + communication). Material cost 5 Euro. Add a margin of 30% if you like to account for your personal gain, costs for storage, homepage costs, tools etc. and we are already well above 50 Euro.
The prices I ask depend quite a bit on what I sell. If it is something I made for me, which I don't want to keep, then its either a gift or you buy me a beer or two (i.e. 10-20 Euro range).
Sometimes I work on comission. Then you will get something custom fitted for you. But you certainly won't like the price tag as I have absolutely no intention nor motivation to make quick crappy slings in bulk which I can sell at a price people would consider buying
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I also think that quality or, when considering a reproduction, the attention to detail, should influence the price.
For a good quality sling made by a skilled person 70-80 Euro is a decent price I would consider paying.