Yes, the artist in this case is *totally* wrong. You just can't quote one price then expect someone to be able to pay up more, even if you do do twice the work or whatever. Who knows if that person has the money?
It seems like they wanted to "surprise" you and expected you'd be so overwhelmed with happiness you'd double your offer. IDK. Not the proper thing to do.

A piece of advice, though, usually saying an artist has "talent" and "can easily make money" can be a bit insulting, even if you meant it as a compliment. Mostly because a) you don't really know how hard they worked on the piece (if it took them all day, or more, $30/day really isn't a good wage at all) and b) the word "talent" sort of implies an innate ability that one doesn't have to work at. (I realize english is not your first language and you may not know these nuances, that's why I'm telling it to you) Most artists who are good do have talent, but they have also put in a lot of hard work and practice. An artist with talent and no hard work behind them tends to look attractive in places, but overall sloppy. Anyway, what I'm saying is a much more flattering word is "skill" - and probably just skip any comments on how much money they must be making :P