Build a marginal tax calculator

Haskell, 67 66 bytes

Thanks Damien for -1 byte.

This solution is in the form of an infix function, ?, of type (Integral b, RealFrac r) => [(r, r)] -> r -> b. The helper function, #, does the required calculations while ? serves to handle the IO specifications.

a?b=floor$reverse a#b
((m,p):r)#i|v<-min i m=p/100*(i-v)+r#v
_#_=0

Mathematica 85 82 bytes

Derived from Josh O'Brien's code in R.

d_~f~i_:=Tr@Thread[Differences@((i~Min~#&/@d[[All,1]]~Append~∞))d[[All,2]]/100.]

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Usage

f[{{11474, 15}, {45282, 20.5}, {90563, 26}, {140388, 29}, {200000, 33}}, 393216]

108357.

05AB1E, 35 24 bytes

vy³ï‚{0è)˜}³ï‚˜¥²ø€PTn/O

Explanation

vy                         # for each amount
  ³ï‚                      # pair with taxable income
     {0è                   # get min
        )˜}                # add to list
           ³ï‚˜            # add income to the end of the list
               ¥           # get deltas
                ²ø         # zip with tax rates
                  €P       # map product on each pair of [amount in tax bracket,rate]
                    Tn/    # divide by 100
                       O   # sum
                           # implicitly display result

Try it online