There IS a market and they ARE expensive, but that's because they're a fad not because they're a nice or quality animal.
Any 8 year old broodmare is worth only as much as her paper, and the foal, yes may be valuable *due to popularity* but that's not the same as a truly valuable animal. Yick, personally would NOT want to encourage those fads. It's a grade foal really.
If you like the mare and are willing to put in all the resources and care until weaning (and do start marketing the sooner the better) I would pay what you think THE MARE is worth and not too much more. $5/6k IF she is well handled and a good prospect AND has good quality and papers for breeding (that IS what you're buying after all a broodmare, and her worth will effect the foals worth) I would think.
Personally I'd pass completely. Draft horses tend to be sweet but I don't see why people buy them for pleasure horses over a smaller horse unless they're REALLY set on a draft. As said EVERYTHING is more expensive, plus they aren't designed to be riding horse. Even if you really want a Clydesdale THIS one sounds like an awkward situation. Buying a mare in foal with plans pre purchase to sell the foal is just begging for something to go wrong.
Definitely get a good look at the sire too.