Twitter may cost Elon Musk more than he was expecting, as Tesla investors digest the takeover news.

By the numbers: Tesla shares fell by more than 12% today, or $121.60 per share. That works out to nearly a $21 billion paper loss for Musk, which is the same amount he's personally committed to invest into the Twitter deal.

Be smart: It's not surprising that Tesla stock is down. Investors still haven't been told how involved Musk plans to be with the social network, but know it will take at least some of his attention away from the automaker.

• Moreover, Musk likely will need to sell some of his Tesla stock to cover his Twitter costs, which could put downward pressure on the share price.