I own a few lenses with a maximum aperture of f/2.8. For this question, I'll focus on two specifically:
- A Rokinon 14mm f/2.8, which retails around the $500 CAD mark; and
- A Sigma 24-70 f/2.8, which runs closer to $1,750 CAD.
While the price difference is significant, the Rokinon is both MF only, as well as prime, meaning that design, material, and production costs would be considerably lower -- on paper, anyway.
One thing I've noticed with the Sigma is far less of a need to step down to reduce astigmatism-related blurring -- in most reasonable lighting conditions, it's barely noticeable even wide open at f/2.8.
Not so on the Rokinon, which needs to be stepped down to f/7.1 in brighter environments to get to an acceptable level of astigmatism-based abberation.
What causes such a significant difference in astigmatism blur reduction? The only factor I can think of is diaphragm blade count and shape -- the Rokinon has 6 blades, while the sigma has 9 -- but I'm not at all convinced that's the only thing at play.