cost of a hypothetical sigma 9mm f/2.8?
assuming the manufacturer isn't making it to cover the APS sensor, but only the m43 sensor?
Sigma 9mm would most likely be designed to also be used on Sony NEX. On APS-C it would be equivalent to a 14mm lens, which means very bulbous front element, and no possibility of accepting filters directly.
A difficult lens like that would probably cost at least as much as the 9-18 does.