This may sound flippant, but you should price these things based upon value.
Formulas can only do so much. The game is far too complex to have appropriate formulas to cover every possibility, especially in areas such as magic item. The formulas provided are fine guidelines, but they fall apart in many circumstances.
This is especially true when it comes to granting feats (or class abilities) via magic items. Why? Because some feats are far more valuable than other feats.
Once you decide what powers an item should have, use whatever formulas or guidelines are present in the DMG to give you a price. Then, compare the item to items in the DMG that are prices at roughly that level. If no formula applies, just start to thumb through the mahgic item lists.
Is your item better than items at the price you reached through the formulas? Worse? Why?
Then, either adjust the item or the price until it seems to be about as useful as most of the stuff in the same price range.
In general, items that grant feats should probably be priced between 1,000 gold and 25,000 gold, but that is just a rough estimate for the range. You'll need to play around with things and decide if that range seems right to you.