B-2 Visa Extension Fees

[mughais]

I am applying for B-2 Visa extension for my In-Laws. I am filing a single I539 form with suppliment filled in for spouse.
My question is: 1) Is this advisable? 2) How much fees do I have to pay?
The fees mentioned is US$ 300.00. But nowhere it is mentioned whether the fees is per person or per form?

Any help will be highly appreciated.

Also can anyone send me sample letter for reasons for extension.

Thanks

Mughais