I am a little skeptical cause he is a yellow dog...

but I would pay 400 from what you described...but when we consider raising and wormings/vaccines, as well as training...then realistically I would consider paying a little more...
The dog looks like he has a touch of hound which I see as a good thing in a stock dog...and like others have said I want to buy the whole package, good hog dog and breeding potential from said dog...