When buying mainstream print, I was almost always able to buy the 6x or 12x rate for open buys (meaning one time) as long as the salesperson felt like we were going to end up going in on a regular basis. (And if the ads worked, we always did.)

The only reason I'd see somebody locking you into a contract is if they know their publication doesn't work and are trying to get $ out of you, because a smart publication will do anything to woo a potential ad buyer, knowing that success of the ad = a long term ad buyer.