As the title says nutrients don't always have to cost a lot of money. I have used many things I can find around the house or in the yard to help out my plants. I have even resorted to digging in a few trashcans in my day just to get something to help out in my grow.
Vinegar can be excellant for lowering the PH in my water. As you probably know already the water around here in San Luis Obispo County seems to run a bit high, a little dab of vinegar can get it right back down to where you need it.
Poopis great for your plants. Lot's of Nitrogen(N) can really help out through the vegetive growth phase. It's always good to know your shit in this case. Some is definately better that others. Chickens can be very "hot" and will most certainly burn your plants. It's always good to throw this in a pile and let it compost for several months before using it. The other end of the spectrum is worm castings and bunny doodoo. These can be thrown into your soil right away and can give you almost instant results. Your worm castings can also be mixed into a tea and foliar sprayed directly onto your plants for even a quicker gratification. Out in the middle of worms and chickens can be pigs, cows, horses, sheep, goats, and the list goes on. These should probably put in a pile and composted for a couple months as well, just to be on the safe side. Human, dogs, cats and things that eat a lot of people food should be avoided. Also if your going total organic and you have worms in with your plants watch out for horse manure as it might have a dewormer in it wich could easily wipe out your worms.
BBQ ashesare a great form of Potassium(K). Check to make sure your grabbing it out of a BBQ that has been burning hardwood. Oak is very popular here on the Central Coast. It really helps your plants build strong stems and branches.