These are carefully made transformers and not just "anybody" can make them.
I am not saying "anybody can make them", but they are not the only one that can make them.
If you only have one vendor, then he's got you by the balls. The guy at Dxxxxxx.com (I assume that's who they are) probably knows that this amp becomes more popular and wants a bigger cut of the pie. Keep in mind that this amp is on the recommended component list of a major magazine, and he knows that.
But if Frank has more than one vendors, even just one more, then the price increase might not have been so significant, hence would affect sale less.
Even other vendors who are capable of making the same transformers are getting the same hit from material price increase, they would still offer the lowest price they can to get the business. I do believe there are more vendors out there that can make the same transformers.
Metal price goes up and down, I don't see them decrease the part price when the metal price go down.
I'd say Frank should put the bid out and see.