I don’t think anyone’s going to be able to give you a standard or average on that. I will say, Great Britain is not the hotbed of export sales like Holland or Germany, so I’d expect it to be a bit more “affordable” than a similar type from a more prolific area. $65k would not surprise me, but neither would 25k. A lot will depend simply on what yard he’s at (and his daddy going to the Olympics has zero bearing on the price ).
But I would temper your expectations a bit, there’s a lot more to a successful hunter than style. He may get off the plane and decide flower boxes are absolutely the most terrifying thing he’s ever encountered. And if he’s truly been successful in the 1.20m jumpers he likely will need quite a bit of re-tooling to go around like a hunter.
I would also not assume that he’s seen a bunch of bad distances either. And even when those euro kids do miss, they miss with commitment— butt in the tack and leg on. Generally speaking, that’s not how American kids miss, and a lot of the imports flunk out because they can’t deal with the abandonment.
And finally, I would caution against expecting him to get off the plane and go. It will be fabulous if he does, but also completely acceptable for him to need some time to adjust.
When you say you’re trainer has “one lined up”, are they going to ride it and then decide if it suits you? Or are you going over with them? And why only this one horse? A trip to Europe generally involves trying 30+ head.