I sell hosted Exchange mailboxes at £7 per mailbox per month. Doubt you'll get cheaper than that as the majority of that goes to MS for their CAL.
It's the only sensible way of achieving what you want to do, without risking corrupting a PST. PSTs are a dodgy legacy and the sooner they die the better IMHO. (and no i'm not saying this purely because of the first sentence above - you can buy from who you want)