Oh, to be complete, N accessories max out at 300-400 atk. It takes 1045 exp to hit lv 20 (their cap). You can hit that with 14 Ns of the same element in one go for 1400 gems. That's the dirt cheap budget option.

Personally, I think R accessories (500-600 atk) shouldn't be too hard to farm for mid-level players/lazy farmers. As far as I've seen, you can expect 2 silver chests/3 copper chests per run of layer 2. Coppers will drop Ns, silvers here drop mostly Rs (but I've gotten 1 SR to drop this way). It'll take some time to actually get enough matching R's to max out 1 R accessory cheaply. But it's an option to consider if you don't want to bother with the material costs to enter layer 3/4 and/or you're not strong enough for them.

For myself, I don't think that I'm strong enough to reliably farm layer 3, and I kinda don't actually care all that much yet. I'm thinking of opting for Rs for the time being; maybe eventually accrue enough accessory points to trade for the occasional SSR if I ever settle on an element.
Hell, it's possible that I can go for N's first, then switch to R's. N's are cheap enough as is.