any reason for the GTS4 in particular? Any GTS4's entering the country these days have to come in as SEVS cars, so they can be expensive, maybe around $12,000 to $16,000 landed, complied and registered.
You can probably pick up 15 year old ones that have come in under the old import laws cheaper though. However don't expect them to be of spotless condition if they're cheap, keep in mind GTS4's were most commonly used as commuter cars in snowy areas where AWD traction was needed, and therefore are prone to have more rust than regular GTSt's.