What gives higher experience in smithing?

Does crafting higher-level equipment (Dwemer vs Iron, for example) carry a significantly higher experience reward than lower-level equipment?

No. Making Daedric Armor grants you the same experience as making an Iron Dagger. The quickest way to level is to travel to a town, buy out their iron and leather. Use those to make as many Iron Daggers and Leather Bracers as possible, and move on. Doing the same with gold -- the physical stuff -- is profitable and easy to do with Transmute. Enchanting the results is very profitable. The biggest roadblock on your way to smithing godhood is that dozens of daggers and gloves become heavy quickly.

After doing that a few dozen times, you are now the greatest smith and enchanter in the land, rich as sin, and can make magical armor out of dragons.

(My apologies, but it's true, you know.)

Does crafting higher-cost (more bars, leather) equipment have a significantly higher experience reward, or at least has an experience reward equal to crafting an equivalent number of lower-cost equipment, such as daggers?

No, same as above.

Does smelting bars of higher-level metals such as ebony carry a significantly higher experience reward to lower-level metals, such as iron?

No. In fact, smelting doesn't even grant experience (neither does tanning leather).

One thing that does make a difference is that making new items grants more experience than improving old ones.

(Also, FWIW, the exploit in Oblivion was to hotkey up a bunch of bows and arrows, then fire one arrow apiece and repair ad nauseam. IMO, Skyrim got the smithing part totally right as weapon endurance was annoying, but there should absolutely be incentives to produce better equipment.)

Every new item you make gives you a set amount of smithing skill increase. Every item you upgrade gives you a set amount of smithing skill increase (less than making a new item).

While making iron daggers and leather bracers is the most economical way of increasing smithing and generating enchanting targets, there is a benefit to making the better gear.

Speech increases based on the amount of gold transacted. Buying/Selling, doesn't matter. If you buy good materials, craft/enchant with them, and sell them back... You'll get more gold movement per soulgem than when you work with iron daggers.