With principles of transparency, openness and fairness in mind I'd have to agree that other suppliers' quotations would indeed be worthwhile, however that leaves the regular bulk buy supplier at somewhat of a disadvantage as their pricing and terms are already published. I don't think it would be unfair to suggest that any supplier may care to publish their regular pricing structure and terms up front, that would only be equitable, then perhaps decided by a show of hands when the time comes.
I would note that vendor support and good will has been a feature of past bulk buys, IMO that's worthy of consideration along with the actual cost.