Depends if you mean LHD imports or RHD imports, from looking through the for sale adverts, autotrader etc.. Ive found LHD cars to be VERY cheap, 3-4k under the price of a RHD model, but i have only seen one or two for sale.
With RHD import models (full uk spec), I actually think not *THAT* much, depends on the age of the car your looking at, if you look at ones about 2 years old then ive seen RHD imports at upto 1k less than an equivalent UK supplied car, i think mainly due to no warranty after 2 years. but once you look at cars that are around 4 years old ive not noticed any price difference.
Mark