Liam, this sentence can easily be interpreted as "if you make it for 50€, we`ll have a deal". "I got one" is no correct English and can`t be easily understood, especially if you`re living in Asia. I would have interpreted it that way, too. Also, because you normally don`t reject a proposial that way. It was a bit unfriendly, wasn`t it?

Don´t forget the language barrier and the different cultures.

ChoiJ didn`t make a mistake at all. He did quite the reverse. He was very kind and found a way to make the (cute and nice) model for 50 EUR. He just misunderstood that you didn`t need the model anymore.