I argued about Michael's point at our session yesterday, being of the school that the runner in question only had to pay her own share's worth in taxes.
Michael already pointed out much of my argument - taxes are handled as an abstract system over all the runner's income, even the (rather sizable) part that the IRS is pretty much blind to as it's handled in the shadows. By the same argument, only the runner's actual income should be counted; a bounty that she has to split with 4 people counts only for her share. Abstracting it to work as a flat rate over all income cuts both ways.
In-universe, it makes sense too. The character will have to declare the bounty's income, and pay full taxes on it. The character will also, usually, pay full taxes over any "illegal" income she gained (presumably the IRS doesn't care, or it's instead laundered/hidden away with costs associated with that). However, since she received a large bounty (of which she had to give most away to fellow runners), she now has a large amount of income she already payed taxes over to hide further less-then-legal income. In other words, she can use the fact that she already payed a large amount of taxes to launder the money she'll later receive through running. Mechanically, that ends up the same as her only paying taxes over her share of the bounty.
So that means that, in my opinion, both the mechanical interpretation of the rules as the in-universe explanation thereof supports only paying taxes over the SINner's cut over the bounty.
(It ended up not mattering as we dealt with it another way, but the question will probably come up later in similar situations)