I definitely would. A quick response can mean the difference between life or death. As for how much more I would pay, it would depend on the location, experience and reputation of the vet(s), etc. Plus no barns in my area have this, so I have no idea how much more it normally is. Lol
I would probably pay a bit more, if there were other 'perks' to go with it- ie, shots, coggins, etc. Just to have it on site, if there were no other advantages, I don't know. But then, I board at a barn that primarily trail rides. Maybe a barn with a lot of show horses, jumpers, or other situtaions that may incur more injuries would find it more of an advantage.
I would pay more. Not terribly more because I'm a poor college student, but it would be very helpful if needed. My vet is about an hour and a half away (there are no large animal vets in the immediate area. All of them are all about an hour or so away) and her farm call is $120 alone, so my vet bills add up fast.