I'm not sure if it would be any closer for you than Detroit, but have you looked at flights out of Flint? That is where we have flown out of for our last 2 trips to Disney, and are booked out of Flint again for this July. It is only an hour forty minutes from Grand Rapids, plus the most I have paid for a ticket is $190 including taxes and fees.
I just looked at the Direct Air site, and they only hav dates listed thru the end of September. Plus they only fly on Friday and Monday. Maybe that works for you, but unfortunately, it doesn't for me.