$pay_1_5 = ( $rate / 12 ) *
( 0.75 * $price ) /
( 1 - pow( 1 + ( $rate / 12), - ( $years_back * 12 )));
EDIT
I've just populated an Excel spreadsheet with your original Excel formula and set cell values for E12 to 100000, E15 to 2 and E18 to 10, and Excel is giving me 12500.00012 as the result.
However, if I set cell E15 to 2% (which would be a $rate value of 0.02) I get a result of 690.1009038.
So you need to adjust your rate value from 2
to 2 / 100
... 2% is a lot less userous as an interest rate than 200%