If you both agreed the price was $210 then the final price is $210. Shame on THEM for gaining profit of $90 from YOUR mistake! Yes, you made a mistake, but they aren't being honest with you by not having been like, "Uh, hey, why the $90 discount? I thought we agreed on $210?" you know??

I would bring it up with them, explain your error, and hope they understand. Because if, like you said, the final price of $210 was agreed on, well they really can't go back on that! Yuo can't just unpaint or undraw the portrait and take your time back for working on it, can you?