I don't think you need to spend that money.
Last time (2008), my mom's return flight ticket is 2-3 days late than her I-94
expiration. She applied for the visa again this Aug. I did write a letter
explaining the situation, but my mom said they didn't ask for my letter,
didn't ask anything about this late leave. and she got her visa.
I did tell my mom that if they asked, tell them the flight was cancelled and
we have to reschedule another one.
hope that helps.