In my experience, it isn't necessarily based on distance. When I moved my new-to-me Ericson-32 from southern California to the PNW, I got bids from a half-dozen boat-transport companies, and by far the biggest influences on cost were my timing, and whether or not they'd be in the area at that time.
The ideal case for them is when they have a delivery near where your trip is starting, load your boat, take it to your destination, and have another job to load there.
I had originally looked at an Ericson 32-3 in Montana (on Flathead Lake). The cost to move it approximately 500 miles was upward of $6k. Subsequently moving a boat from SoCal - more than twice the distance - ended up costing roughly half as much.
There are a variety of other factors. Over-height or overweight loads can cost more. Weather delays can jack up the price. But generally, IMO, the bottom line is if the start-point and end-point are on the company's schedule, and your timing is flexible, the cost can be fairly reasonable.
One thing to note is that the boat-haulers don't generally expect to do anything to prepare your boat. They view it as a "thing", they load it on their truck at one boat-yard and have it lifted off their truck at the other end. They don't know or care what's in the boat or how it is protected - their job is to get the "thing" from one place to the other. So boat prep is important, IMO - whether you do it yourself, or hire a yard to do it ($$$$!), the better prepared the boat is before the trip, the better shape it will be in when it arrives at the other end.
More detail here, if interested:
http://www.ericsonyachts.org/infoexchange/entry.php?354-Boat-haulers-a-long-strange-trip
Edited to add: my experience with U-ship was ...uh... less than satisfactory. Putting it mildly. I submitted my info to get bids, and started getting flooded with sales calls, most of which had absolutely nothing to do with moving a boat. I never did get a viable bid through U-ship. ymmv.