There doesn't appear to be a hard-and-fast set of rules in 5e about what constitutes "art object" vs totem or focus. Your best guideline is likely the DMG, specifically Chapter 7: Treasure. The chapter opening divides possible treasure into four categories: Coin, Gems, Art Objects, and Magic Items--the separation is important, implying that Art Objects have no inherent magic, or magical properties, of their own.
Taking a look at the treasure tables on pp. 133-134 backs this up; the available ideas for Art Objects are all items of visible worth (usually involving precious metals, gems, and fine workmanship) but never make mention of any magical capability at all. So it appears that items of fine craftsmanship, without magic to them, can be considered Art Objects and thus retain their value at sale, but items with magical properties cannot--they become Magic Items.
It does appear that items need to hit a certain level of aesthetic expense and craftsmanship in order to qualify, though that may be just the limitation of the reference here. So expect a DM ruling about the relative value of your wooden statuette, presumably taking the quality of carving and the rarity of material into consideration. Rough-hewn abstract carvings made of the region's most common tree probably won't do as well as a meticulously detailed piece of a rare wood from another land.
So, moving on to your second question: when evaluating totems or foci, that leaves us with two main metrics: quality of craftsmanship, and whether they're considered magic or not.
The question of whether basic foci, regardless of class, have enough innate magic to be considered "magic items" in the first place should probably be another question on its own. I would argue it's up to the DM to make a ruling here one way or the other--either a basic focus has some small amount of innate magic in order to enable it to be used as a focus in the first place, or it doesn't, since it can be found on the basic Adventuring Gear table, which has no magic items on it at all. Whichever argument you go with will inform your game and your world, even if only in flavor.
A judgment on the quality of craftsmanship also belongs to the DM here. As druids generally eschew a lot of the high-expense materials found in art objects, druidic totems are subject to the same criteria as your wooden statue: craftsmanship, and innate magical properties. You could make more of an argument for a Holy Symbol of an amulet or reliquary to be "blingy" enough to be considered an art object, considering the long, real-world history of people spending a lot on religious icons to prove their piety.
TL;DR, Art Objects need to match two criteria: high level of craftsmanship/expensive materials, and not being magic. Consult your DM for specific rulings on a particular piece.