If you assume that 40 employees - a close knit team - works out at $1mm in value per head over 4 years, that is $40mm. If you consider that Draw Something is making, say, $200,000 a day in revenue - that is $1.4mm revenue a week. $5.6mm revenue a month. $67mm revenue a year. That is for the first release, what if this is the next Angry Birds? imagine the value of that audience to up-sell new titles, new micro-transactions, and so on.
Suddenly it looks cheap if you consider the massive popularity of Draw Something, this is a game that everyone loves! For Zynga, a publicly traded company, this matters. It's important to look like you're responsive to potential killers (mobile is something Zynga does not focus on much yet, but need to.)
And we haven't even mentioned the fact that OMGPOP are OMGHOT right now! Imagine the premium you'd have to pay for this company, they must be flooded with VC term sheets ("yo raise $40mm from us and give yourselves $20mm.") OMGPOP can turn this into the Zynga of Mobile. We've already valued the company at nearly $100mm from the team and singular game, a 2x multiplier doesn't seem ridiculous when you think about it.
OMGPOP will be to Zynga what the YouTube acquisition was to Google: huge.