http://www.mobileworldlive.com/viber-agrees-900m-sale
This should help establish a base for a valuation of BBM.
Viber claims to have 280 million registered users, but only 100 million of those are presently active, according to the above article. Notice too that Viber has a voice function that allows users to make low-cost calls to landline phones or mobile handsets. The article is not as clear as it could be, but Viber started to add revenue for the first time just recently, making $1.52 million from a "sticker market," but it was losing money ($29.5m in 2013).

Next, compare Viber's download share per country compared to BBM's: http://www.blackberrycool.com/2013/12/04/the-market-share-race-is-on-bbm-vs-snapchat-vs-whatsapp-vs-kik-vs-viber/
BBM is at least twice as popular as Viber in four out of five markets in the survey just cited.

On this basis, I'm going to suggest that BBM is worth at least twice what Viber is worth.
$1.8 billion. At the very least, it's worth the same amount as Viber, or close to $1 billion.