In our opinion, swap items are there to be swapped, like for like. After all, we're here for the sport of the game, not to put a price on the contents of a box! If we costed time, petrol etc., we would never go caching!
TBs should definitely be a separate entity, and not classed as a swap item - they're there to be moved on as swiftly as possible and you are still playing the game fair and square.