Earlier this year (after the run-up in gas/diesel prices), we spent $1,700 transporting a 34', 12,000 lb boat approximately 350 miles between Cleveland and Chicago. We had a lower quote (approximately $1,500) and several much higher quotes (the highest was approximately $2,400). Based upon the foregoing, some of the transport costs I've seen guessed in previous responses are probably not to far off.
As an aside, we used the company recommended in the prior response (M&N Transportation) and were pleased with the service. We had regular updates from the driver and no damage. I should mention that I have no relationship with M&N, economic or otherwise.