curious of this too.
vistaprint 250 5x7 inch card (front + colored back) = 102 $ + tax
they only have 100 and 250 cards options
US stamps 0.35 * 250 = 87.5
guess you can pay a friend you trust in US to send them for you? for ~$100 or what ever you both feel comfortable. It's just writing address, put stamps and dropping them to USPS.
So you end up paying ~ $300 or less for 250 cards. or if you are willing to do 100 cards with black and white back, it will be much less.
BTW, love your art!