If you find a great deal on the cross frameset of your choice, go for it. I wound up getting my conquest pro brand new on ebay for $280. After swapping out parts, and selling the Bianchi Vigorelli frameset they came off of, I broke even. This is a miracle considering the extra cross specific parts and labor costs for certain assembly projects I did not have the tools to complete. The end result was a lighter bike with better components than stock for $100's less than retail in a shop.

Also, I should think that your Tarmac frameset would fetch a much higher price on the open market than my Bianchi, leaving you even more money for additional upgrades to your bike and/or equipment. I could be wrong, but in my experience some shops are similar to car dealers in the low amount they offer on trades.