Dear [insert name],
Thank you for the invitation! I would love to join you for [insert event]. Unfortunately, I've recently discovered that my bank account has a giant crack in the bottom of it and all I can hear is a sucking sound.
Please know that I would love to be there, but until this economy turns around, I am forced to live off of [an embarrassingly small amount of money] per day.
[Anyone currently living in America who use to call themselves middle class but is now classless and/or poor]