If you know well ahead of time when you'll be there, you can very likely find somewhere free to stay via couchsurfing.org, or cheap via hostelworld/airbnb. Good luck!
Couch-surfing in Hawaii, at least on the Oahu/Maui/big island is a dead-end. They get flooded with people who don't really care about couchsurfing; they just want free accomodation. Consequently they generally ignore requests, especially from newbies. This is how it was explained to me.
You're not going to find cheap accomodation in Honolulu. I don't know if they have any hostels. Maybe North Shore, if you like to hang out with surfers. If cheap is the driving goal, options are:
1) a $50-100 hotel room on the remoter parts of Maui or Big Island (they do exist)
2) a hostel (kinda trashy IMO)
3) An AirBnB room
You'll want to avoid Honolulu if you want cheap.