notretseh, your family income must be fairly high for Northwestern to have been awarded only $3500 in financial aid. According to my Northwestern NPC estimate, a family making $180K (with $90K cash/checking + $90K investments + $90K home equity, not including retirement savings) would get over $13K in grant aid.
Is Northwestern’s aid offer much less than what the NPC would have predicted? If so, try bringing that to their attention and see if they will increase the offer. Be sure the award was based on complete, accurate information, if there is any doubt about that, and that the entire financial aid package already has been decided.
If the offer is approximately in line with what the NPC would have predicted, weren’t you aware that more aid was unlikely when you decided to apply there? If you applied ED, that should have been with the understanding that you would accept an offer from Northwestern as long as you could afford to attend (even if it meant passing up scholarships from other colleges.) The mere fact that Purdue is ~$7500/year cheaper (“I would have an extra $30k if I went to Purdue”) is not necessarily a valid justification for declining an ED offer. If Purdue finds out you reneged on an ED commitment at another school, without being released from that commitment due to inadequate aid, then at least in theory Purdue could rescind its offer. Whether they actually would do that, I don’t know … but to be safe, you better talk to Northwestern and get released from the ED commitment before accepting any other offer.
http://www.ugadm.northwestern.edu/apply/application-options.html
“Students admitted under Early Decision may be released from the commitment to enroll at Northwestern only for demonstrated financial hardship.”
(I’d expect them to be pretty generous in interpreting “financial hardship”.)