I held some ANDV stock that got bought out by MPC. Terms of the deal were approximately 1.626 shares of MPC per ANDV share, plus $19.80 cash per ANDV share. My 1099B is showing a gain of $1,643.32 (that's the cash payout; no shares of the new MPC were sold) on 83 original shares of ANDV, but the broker is reporting my cost basis as $0, meaning I'll have to pay more in taxes.
In reality, I believe my cost basis is really ($19.80 / $152.27) times my ANDV cost basis. (Where did I get $152.27? That was the original cash consideration per ANDV share, but I elected all shares; since there weren't enough shares, ANDV shareholders got part MPC shares and part cash)
Is it worth it to report a basis to the IRS that differs from the basis reported by my brokerage ($0)? I'd like to avoid an audit at all costs due to the hassle involved, so if this makes that more likely then I'll probably just suck it up and use the $0 basis, as the difference is only about an extra $100 in federal taxes owed.
(Aside: Does the IRS give you firm guidance in the form of a formula for how to determine basis when you're not actually selling any shares but rather receiving shares in a new stock, plus a lump of cash? Or do they leave it up to the taxpayer to apply common sense?)