I pay $8 for hair dye at Walmart. I do my roots with it. It lasts me 4 applications or about 6-8 months. I use a scale and measure out the grams of both ingredients. I have had really good experience with this L'oreal dark auburn hair dye.
Watch some youtube videos on doing your own roots, it's prtty easy. Can train significant other or child to do it for ya based off the vid. Works great. Really easy to do. Just comb through after 20 mins or so, to blend the colors from roots out. Then wait another 10-15 mins and rinse.
My hair doesn't get greasy fast, so I shampoo once every other week.
The first time I did my own roots, I did it myself in the mirror. Was easier the subsequent times after I showed my boyfriend what to do. Just keep parting and brushing the greys.. after 20 mins or so comb through and wait another 10-15 mins, then rinse.
Tools needed: disposable gloves (I use surgical nitrile), hair dye bowl, hair dye brush, rat tail comb for parting. I put a plastic grocery bag over head otherwise you can get those caps. I bought all this stuff at Sally Beauty Supply. The gloves I got from Amazon.
Another idea, if your family / friends don't want to help is you could barter on craigslist / facebook marketplace. Could offer to regularly do their roots if they do yours. I haven't done this yet but have considered it because my boyfriend can get pretty pissy at times about it :P.