I'd say you best recourse in the one is too search for decora pics you like. And then you work it from there. Just as peachpudding said, decoras make lots of stuff themselfs. It's a very colorful and happy lifestyle. An idea to wear everything that makes you happy. Kids sections are a way to start. They got some nice tutus sometimes, not to mention they got millions of cute hairclips and such ^^ (also check the toystores) You can also find cute pouches and such that decoras often carries around their neck.
General cost is such a difficult question. How much are you willing to spend? I'm into lolita, but I'm willing to spend every coin that I have on lolita because its THAT important to me. Because i love it so much. Money is such a small part when it comes to fashion. I can't afford any brand, but that doesn't make me less a lolita than everyone else. So you spend just how much you wanna spend. There is no end when it comes to develop and evolve a style :)
Decoras don't use a specific materials. They grab what they find and what they like. The more colors and patterns the better. Layers of socks for instance :) And of course different socks on different feet ^^ Decoras are playing with colors and layers and there is nothing really right and wrong. If you find a nice one color shirt/sweater you can add some patches easy with an iron and a few stitches to make it more happy and decora :) Patches can be found in any sewing store i believe (or ebay if you want some cooler ^^)
Good luck!