Howzit
Put the formula in the control source of text box 6 and it will update after each entry. Cater for null values by using the Nz function. Something like...
=((nz([TextBox1],0) + nz([TextBox2],0) + nz([TextBox3],0) + nz([TextBox4],0)) * (nz([Labour],0) * 40) + 60) * 1.5