Those on the run or thinking of running will appreciate these links. Just because you're homeless doesn't mean you're helpless. There are many places that will buy scrap metal and other useful bits if you have the time to clean them up or are recycle-crafty.
Consider Urban Foraging as an easy way to earn cash on the run. That really is an incredibly useful link, with links to other sites as well. There are too many sites to list on the subject of recycling garbage into crafts, and if you're not greedy, you can sell these to people for spare change without getting into trouble for grifting. Just a few tool (needlenose pliers, epoxy, nailpolish) can allow you to craft small decorative bits on the run.
Urban Food Foraging is a good way to make do with your meager finances. Though Autumn is often considered the best time to find ready food, Spring is no slouch in offering readily harvested edibles as you move from area to area. Edible buds, shoots, and roots can provide a meal when nothing else is available. Dumpster diving at grocery stores (careful of trespassing and bloated cans) or even asking restaurants for free food in exchange for a night of dishwashing are easy ways to get food.
You'd be surprised what foods are edible in a pinch... I say edible, not appetizing. There are lists of famine foods compiled during the Depression and WWII that give detailed instructions on how to make even some plants considered poisonous edible. A quick search online should give you an idea of preparation. I also recommend Grieve's "Modern Herbal" which includes medical, practical, and famine instructions on various plants, though it does have a British bias and only sketched pictures (if you have access to the books). Any plant identification guide with nice glossy pictures is a good thing to have around as well.