Most of the difference is explained at Why are some lens hoods petal shaped and others not?, with the remaining question being the cost.

And, I don't think the basic cost premise is correct. See cheap tulip hoods at B&H, where they start at $4 -- a dollar less than the cheapest circular hood. So the answer to "why are tulip hoods more expensive?" is... "they're not".

The Canon / Nikon branded hoods are more expensive, but even then, there's no real difference based on shape. If you take a look at Canon or Nikon lens hoods sorted by price, you can see both shapes mixed in at different price points. While it does happen that the very cheapest models for SLR lenses are small rings of plastic, as you go up, there is no general pattern where one type is cheaper.

In fact, the most expensive lens hoods (Canon, $700; Nikon, $1000) are just tubes with no petal shape. These hoods are for extreme telephoto lenses where the tulip shape provides no benefit (see the other question), and it's clearly supply and demand (particularly, rarity in this case) which determines the price. That's true at lower price levels too.