Ahhh .... yes !
Confirmed in foal, not exposed, "I have not seen her come back into heat", possibly, maybe, could be, stallion got away from me one day, etc.
I have over the years bought a few incredible mares that were in foal to an outstanding outside stallion and had awesome foals unrelated to my herd. Some of which I still own. Of course it is great when they are in foal to a well-known stallion, but if it is a beautiful, not shown stallion, I personally do not care. I am looking at the horse, not the show record.
I do not expect a breeder to sell a CONFIRMED IN FOAL mare to me at an open mare price. Get real !
Only the Buyer can decide if the mare and the stallion she is bred to fits into their breeding program.
IMHO you should ALWAYS buy the mare for her own sake, not for the foal she is carrying, as you never know if she will have a live foal. There are too many variabes.
And, if I see an amazing mare for sale that has an amazing stallion at their ranch, I may inquire for her to be bred to this stallion in hopes of adding some new blood into my breeding program. That is just good breeding strategy.
And one more thing, If you buy a mare in foal, make sure you hire a transporter that knows they are in foal and will take excellent care of your mare.
I personally HATE a pregnant mare being moved late in pregnancy. I think it is unfair to the mare to be jerked around at the end of her pregnancy.