The Pearl 3G is on average $329 full price (Telus is $200), but it is usually subsidized to $0 on a full contract.
Higher end phones ($600ish) vary more, but I do see some for free as promotional pricing or something.
A PlayBook 16gb and a Pearl/Curve 3G would total about $ 829, minus a possible subsidization of $600 means it's quite possible to get a PlayBook and a low-end Blackberry with an up-front promotional price of $229ish on a full contract.
Looking at Telus only the average subsidization for a high-end phone is $500, which could give you a PB+Pearl/Curve for $329 on a full contract.
I do think it would be quite an attractive thing to do for a carrier. They could use this tactic to clear out older RIM devices in stock and to sell tethering plans at the same time. It would surely sound pretty attractive to a lot of people and the fact that the device is bundled with a BB device would eliminate the biggest issue consumers are likely to have with the PlayBook: the lack of email/pim/messaging. They'll have it!