isn't there a discount on xbox live for DG + HF at the moment? if that's the case, i think it would be fair to introduce the "old" dlc with a reduced price.
I wouldn't know for sure, but the DG bought seperately is still the equivalent of $20 USD.
I know Steam had a DG+HF sale for for %20 off, which for $20 you essentially got HF for free. That has since ended and was a Steam sale, not made by Microsoft or anyone else.
Remember what the PSStore, XBMP, and Steam actually are: Retailers. Retailers can control when they do bundles and sales, but of course manufacturer can deem them too. Bottom line, the price of the individual dlc's is still their original price of $20 and $5. The bundle that came out was a retailer bundle, at least on Steam as I don't recall ever seeing that deal on XBMP.
As I said, if Bethesda authorized a sale at any time that would lower the price of content, it should be for everyone. They haven't yet.