I have heard DT is insanely high in cost.
Frightful fiend was pretty high and I think was easily the most expensive bard edge, but I don't think it compared.

I think the most expensive is actually devious versatility, just because it can be taken repeatedly, but the cost increases each time (I believe it does at least)

I think it was so expensive that no character could ever take it 3 times in the old edge point system, even though it was technically available. Not sure how the second DV compared cost wise to demielemental transcendence