There is no simple (official to my knowledge) answer.
When magic is involved the cost is given by the magic enhancement irregardless of size. As money quickly goes into the tens of thousands, few care about tiny sized weapons base cost instead paying full price and saving time on the paperwork shortcut.
However, there are some suggestions:
- DMG has section about modifying equipment for abnormal body types, such as wings or extra arms. Its 30% more base cost. Considering the difficulty in making tiny rivets and stitching...
- Model the solution off of armor: Tiny, Fine, & Diminutive = 1/2 x Small. Large x 8 = Huge x 4 = Gargantuan x 2 = Colossal (doubles each size increment). The larger options being for material costs, while smaller requires crafting skill regardless of material savings.
- Outside of weapons: magic items (like rings) adjust to user size and have the same cost and weight. Personal items (like tents, food, water skins) adjust by 3/4th per size increment by Player Hand Book. Some items (like tools, vials, books, and rope) have a fixed cost regardless of character size, by logic and player hand book. [Huge troll climbing thin silk rope, maybe house rule that?]
- In general, the answer is always ask the DM, house rules abound. Some campaigns ignore weight in all but extreme cases. [Carrying a castle or 100 weapons.]
The text on the Tents is specific about this.
Tents come in a variety of sizes and accommodate between one and 10 people. Two Small creatures count as a Medium creature, and one Large creature counts as two Medium creatures. Packing up a tent takes half as long as assembling it.
There is no specific rule for feeding creatures of exotic size, however this information can be extrapolated from the tent description and other rules such as the ones for trail ration and other food items that mention that the version designed for Small-Sized humanoids weights only 1/4 as the original, which should translate to 1/4 of the content. This also matches the text on enlarge eprson that says that 1 size category increase is "double height and quadruple weight".
For other effects related to size, there's a lot of effects that mention how to handle this, for example, the text on Feather Fall says:
The spell affects one or more Medium or smaller creatures (including gear and carried objects up to each creature’s maximum load) or objects, or the equivalent in larger creatures: a Large creature or object counts as two Medium creatures or objects, a Huge creature or object counts as four Medium creatures or objects, and so forth.
By this, tiny creatures would count as 1/4th the size of a medium one.
There's several other examples that use this "rule" for handling creatures of unusual size so it can help as a "hard and fast" one in case nothing better is specified.
As for feeding animals, you can use the "Feed" entry in Animals & Animal Gear of the equipment. The cost depends on the diet of the creature, but here's an example:
Consisting of various kinds of raw meat that have been jerked, smoked, or salted for preservation, a day’s worth of carnivore feed is adequate food for any Small or Medium meat-eating animal such as a tiger, bear, or dog. Larger carnivores might require up to 2 to 4 days’ worth of carnivore feed per day.
Finally, for the saddles, every creature that is not a "standard" riding animal like a Horse or Pony should use Exotic Saddles, which have a fixed prize.
Best Answer
It's up to the DM's discretion.
Notably, a light hammer is a simple melee weapon that deal 1d4 bludgeoning damage and has the light and thrown (20/60) properties.
Per Improvised Weapons (PHB page 147),
So if we assume that the hammer in the adventuring gear table is not a light hammer in the weapons table, then the only real difference between the light hammer and an improvised weapon is that, for ranged attacks, the light hammer can use a character's Strength modifier for the attack roll and damage, while the improvised weapon would need to use Dexterity.
The sledgehammer would either be an improvised weapon, or looking at the weights (which is not a criteria for the DM to consider), a greatclub or maul. The DM could also just choose to let the sledge hammer represent any other weapon.
The hammer is also a component in several tools, according to the optional rules in Xanathar's Guide to Everything: Carpenter's Tools, Cobbler's Tools, Jeweler's Tools, Leatherworker's Tools, Mason's Tools, and Smith's Tools. For Jeweler's Tools it specifies a "small hammer" and for Leatherworker's Tools is specifies a "mallet", but I have included them in this list for the sake of completion.
The hammer is additionally included in the Burglar's Pack (PHB page 151), so it would be reasonable to conclude that they are used to destroy small or fragile objects, such as a window.
This next section is not based on game mechanics, but just describes how a hammer might be used by real-world artisans. With Carpenter's Tools, the hammer seems to be used to hammer nails into place, and as a way to apply force to a chisel to remove wood. The Cobbler's Tools and Leatherworker's Tools seem to have included it as a way to compress or flatten the thread and leather. For Jeweler's Tools and Smith's Tools, the hammer is used to shape metals (cold for Jeweler's Tools, and hot for Smith's Tools). For Mason's Tools, the hammer seems to have been included as a way to apply force to a chisel to chip away stone.
Per Objects (DMG page 246),
Per the Sledgehammer Wikipedia article,
Ultimately, the Adventuring Gear section on page 148 of the PHB may say it best,
So by virtue of the fact that the hammer and the sledgehammer lack special rules or further explanation, they function as they would in the real world, at the DM's discretion.
With this in mind, I think a common sense ruling would be that a hammer functions as a light hammer as described in the weapons table, and a sledgehammer can be used to demolish structures made from wood or brick, but not necessarily natural stone formations such as cave walls. That is common sense to me, but may not be common sense to another DM, so as usual it's "per DM's option" on how effective these pieces of gear are in any given situation.