I think the final price of the Cubieboard is $50, the cheapest SSDs start at around $50 too [1][2], so yeah it's going to add to the price, but it's also going to make it useful for a lot more projects than an SD card.

1 - http://www.newegg.com/Product/Product.aspx?Item=N82E16820227...

2- http://www.crucial.com/store/partspecs.aspx?IMODULE=CT064V4S...