I think I am not the first, who , after finding a nice piece to transmog, and looking up how to get it, it became clear, that the receipt is no longer in the game, becasue the source was removed, in cases with the source of the recquired materials.
And with it, the prices are beyond imagination, seems like the seller (who has the receipt) try to cover their monthly subscription with 1 sale.
For example the Pattern: Blue Dragonscale Shoulders (https://www.wowhead.com/classic/item=15763/pattern-blue-dragonscale-shoulders#teaches-recipe) fall into this category, it costs about 270k on ah.
My suggestion is, that the unaviable receipts shall became aviable again, either with a token-vendor system, or reintroducing them as possible drop from vanilla era bosses