Since $\text{CPR} = 1 - (1 - \text{SMM})^{12}$

$$\text{SMM} = 1 - (1 - 0.06)^{1/12} = 0.005143$$

Prepayment = $\text{SMM} \times (\text{Outstanding principal less scheduled monthly principal payment})$
\quad = 0.005143 \times (\`100,000 - \28.61`) = \`514.15`$

Things to remember.

SMM is a measure of the monthly mortgage prepayment rate of the security’s mortgage pool. A SMM of 0.5143% means that approximately 0.5143% of the remaining mortgage balance at the beginning of the month, less the scheduled principal payment, will prepay that month.

In other words,

$$\text{Prepayment ith month (\$)} = \text{SMM} \times (\text{Principal Bal. i} - \text{Principal Pmt. i})$$