Co-op grade-B vanilla is less expensive because your order is bundled with other orders to save on costs. Orders with products labeled "co-op" usually ship in 4-6 weeks.
Retail grade-B vanilla is more expensive because it is warehoused and ready to ship. Retail orders that are not coupled with co-op orders will ship in 2-5 days.
Grade-B vanilla beans are much drier than grade-A and often brittle. They are a great bean to use in homemade vanilla extracts, vanilla powders and vanilla sugars and not much else. They will be less oily than grade-A with fewer cooking applications.