This is a bit complicated because of all the moving parts, but is a little simpler because the two warrants are now publicly traded. The main rule appears to be that your cost should be apportioned into the bases for the pieces you received by the proportions of the prices established in the market on the first day of trading in which they trade separately (source: costbasis.com). Since the A and B GM warrants began trading in March 2011 (at least that's what a quick search shows), use their prices and the GM price on the same day to establish the proportions. You also must include the factor of how much of each piece you received for each of your bonds.

So, for example, if the prices of GM, WSA, and WSB, were $32, $23, and $17 on the first day of trading, and you got 3 shares GM, 2 A warrants, and 1 B warrant for your bonds, their worth on first day of separate trading would be:

w(GM)  = 3*32 = 96
w(WSA) = 2*23 = 46
w(WSB) = 1*17 = 17
total          159

and so the proportion of your bond cost to be allocated to your A warrants, for instance, would be 46/159 or about 28.9% using these example figures.

The small dribbles of additional securities you have received already, I would include in the calculation above, and if you in the future receive any further dribbles, I would assign them a basis of $0 (as your full bond cost would have already been completed allocated).

This is a bit complicated because of all the moving parts, but is a little simpler because the two warrants are now publicly traded. The main rule appears to be that your cost should be apportioned into the bases for the pieces you received by the proportions of the prices established in the market on the first day of trading in which they trade separately (source: costbasis.com). Since the A and B GM warrants began trading in March 2011 (at least that's what a quick search shows), use their prices and the GM price on the same day to establish the proportions. You also must include the factor of how much of each piece you received for each of your bonds.

So, for example, if the prices of GM, WSA, and WSB, were $32, $23, and $17 on the first day of trading, and you got 3 shares GM, 2 A warrants, and 1 B warrant for your bonds, their worth on first day of separate trading would be:

w(GM)  = 3*32 = 96
w(WSA) = 2*23 = 46
w(WSB) = 1*17 = 17
total          159

and so the proportion of your bond cost to be allocated to your A warrants, for instance, would be 46/159 or about 28.9% using these example figures.

The small dribbles of additional securities you have received already, I would include in the calculation above, and if you in the future receive any further dribbles, I would assign them a basis of $0 (as your full bond cost would have already been completely allocated).