Just so Roche execs will know how much it will cost them to hold on to all those Genentech staffers, the biotech filed a document with the Securities and Exchange Commission yesterday showing retention bonuses are going to add up quickly.

And the retention plan would extend to virtually all of its 10,700 employees, including, of course, ceo Arthur. Levinson, who would be entitled to a bonus of $8.7 million.

Roche, which owns 56 percent of Genentech, offered last month to buy the rest of the company for $89 a share, or $43.7 billion. A committee of independent Genentech directors rejected that offer last week but said it would consider a higher price. Meanwhile, as we noted yesterday, headhunters are actively recruiting Genentech employees, potentially robbing Roche of valuable assets.