You have to contact a kit vendor for the price. Check with Mattituck, Eagle Engines, A.E.R.O., or some of the others.
Here are the prices I received in September of 2009 when I was shopping for my kit:
(Note, the HP and weights came from the ECi site at that time. I will not divulge which vendor gave me those prices, so don't ask. Besides, they are sure to have changed by now.)
Engine___Price____HP___Weight
O-320__$14,200__164___270 lbs
O-340__$14,800__177___277 lbs
O-360__$14,700__185___282 lbs
Those numbers do not include a carb or fuel injection, fuel pump, ignition, starter, or alternator.
When you order yours, make you specify a hollow crank and then tell them if you want it set up for a CS or FP prop. I missed that last bit of info and had to punch the inner seal and set the outer plug.
I also ordered the cylinders with the tapered cooling fins on my O-360, which saves about 1.5 lbs each. Thus, my O-360 should have weighed 279 pounds.