Why do lens manufacturers produce lenses with few aperture blades?

A lot of lenses have very few aperture blades (especially budget lenses). A large number of aperture blades can make a real difference to the bokeh and seems like it should be a priority for the lens manufacturers to me. If the cost of adding a few more blades is low it would seem like a cheap and easy way to improve the value of a lens.

I get that a more intricate design will be more expensive, but will the addition of a few extra blades really add a lot of cost to the whole lens? Is it mostly a marketing trick perhaps to add apparent value to the more expensive lens alternatives? Is there another reason I've missed perhaps?