Simply get an estimate of your monthly PAYE contribution:
These are based on SARS published tax tables, but as always, the definitive answer will be the one SARS gives you when you submit your tax return. Also, if you have a slightly more "complicated" setup, I would strongly advise you seek a reputable tax adviser.

Taxable Income
  Is Annual Income
Aged under 65
Aged 65 or over

Taxable Income PAYE Take-home
0 0 0
0 0 0
Download PAYE Javascript library
Download (right-click 'save as') unit tests for JsUnit