The problem with Shadowbolt Dash's price is that only hard core collectors are going to be willing to spend that much on one pony. And usually collectors either hold on to them forever, or if they do tire of ponies they will try to sell them off and make a profit. There will likely not be any yard sale or thrift store luck with Shadowbolt Dash.
I don't know about part of that statement- as pointed out by the U.K. folks, they are already paying that amount for a regular pony! Sales don't seem to be suffering in the non-U.S. market despite the price difference.
But I agree that it is very unlikely that she would turn up in a Goodwill bin or thrift store (actually ponies in general no longer seem to turn up at the thrift store- I think their sorters are getting better)