DIY C serums can be easy to make. C serums are expensive because it is hard to keep the C stable. If you make it yourself, you know it is fresh. You can mix it daily quick and dirty or you can do more measuring, use a few more ingredients and mix it weekly.

For daily you can just mix l-asorbic acid, and water. Stir for about a minute, apply. This is what I do, although I've been lazy lately.

For weekly, the recipes I've seen add propylene glycol and use grams. Store in the fridge.

If you want more info, lmk and I'll post the links I have to recipes.
http://pedaheh.blogspot.com/ (a blog to help new wavies, go to the how do I get started page)
2a (nc.com) 2bFi (Fia), very fine, thin, low/normal elasticity, porous, CG, doesn't get weighed down
Cowash: Tresemme naturals
Rinse out/leave in: Renpure Organics (red)
Curl enhancer: FSG
Gel: BRHG, Salon care aloe, Ecostyler Krystal (normal dews)
PT: ION effective care, IAgirl's gelatin PT
Low Poo: renpure (red), Ion swimmers