I wouldn't ride that horse too far because there's a substantial amount of knowledge being given away on GS every day by people in business. The JW mod is no secret because he posted it. It's also no secret that the opamps and capacitors are the common subjects of the mod. Since it is already known the opamps and the capacitors are the subjects of the mod, it's not big step to disclose which ones are being used or at least give some more specific indication of what the mod consists of.
So far as adding $22.92 to the production cost of the DR70d, we have seen a price swing of $299 to $179 since it was released about October of 2014. Tascam apparently has some room here to have used higher grade components, and then they would have had an even better product.
But as priced now, it's $300 MSRP plus $150 for the mod. That's $450 instead of the $325 or $350 it could have been direct from the factory before discounts--possibly $205 to $225.
Joshua-G's writing is excellent, BTW.