They would go up because, unlike American made products, who would be relieved of the Federal tax burden they bear under the FaitTax throughout the entire production tree, prior to having the burden put back on them at the point of sale, imports have no tax burden on them now, so there is nothing to be relieved, but will still bear the burden of the FairTax. So they will have a net increase in purchase price of 30%.
The good thing is because it will reverse some of the negative effects the cheap chinese imports have had on our economy. Particularly, because it will tend to regain some of the marketshare lost to cheap imports, it will allow American goods to come down in price, and become even more competitive.
Less than that, I'm sure. Even imports have to be shipped, warehoused, marketed, sold, etc. -- those are all domestic activities that would be subject to taxes (and associated costs) under the current system.