Compost tea, it's cheaper than any nutrients you can buy once you get past the start up costs which aren't that expensive themselves. Here are all the recipes you need in the post below
https://www.rollitup.org/organics/510995-organic-feeding-101-a-2.html#post7914959
Just a word of caution though, it says use 5TBSP of molasses but that can be too much sometimes. Just start off with 1-2TBSP and see how that works for you. All that you'll need to do is go to your local Home Depot/Lowes for some 5 gallon buckets. You'll need a pump that's powerful enough to pump through the amount of gallons you'll be making. I have two 5 gallon buckets, one for my veg and flower teas respectively. I spent $30 for an aquarium pump that's supposed to be good for up to 20 gallons but it barely does the job. I would recommend spending a little bit more money on a hydroponic pump, these pumps are built for our hobby where as the aquarium pump I bought is just meant for aquariums. You just need something powerful enough to constantly pump air into your 5 gallon buckets of tea.
Another word of advice, go to your local pet store and look for a dechlorinator product. My local Petco has this bottle of stuff that's for aquariums, it's about $10 for a nicely sized bottle and you only need a tiny amount to remove the chlorine/chloramine that is bad for your beneficial microbes. Plus it's made with aloe vera which is also a great organic ingredient.
The ingredients for those tea recipes are usually very cheap though. You get a 2.2 lb box of the guanos, alfalfa and kelp meals, and most of that stuff for $7 bucks each and they last for quite a few batches of compost tea. And instead of earthworm castings I like to use the alaskan humisoil because it's loaded with microbes that will make your tea great. Make yourself a veg and bloom tea asap with these recipes and you will be completely amazed with the results, you usually see results within the next day.