As of Skyrim's 1.5 patch, you get differing values for smithing items in part due to their value. More expensive, more experience.
UPDATE 1.5 (all platforms unless noted)
NEW FEATURES
New cinematic kill cameras for projectile weapons and spells
New kill moves and animations for melee weapons
Shadows on grass available (PC)
Smithing skill increases now factor in the created item’s value
Improved visual transition when going underwater
Improved distance LOD
transition for snowy landscapes
Emphasis mine. So what's the skinny? Are Iron Daggers and Leather bracers still the best way to skill up, or will I actually be better served with my hundreds of Dwarven Ingots?
Best Answer
According to UESP, formulas for XP gain post 1.5 update are as follows:
25 + (3 * item value ^ 0.65) base XP for constructing an item. 25 + (8 * item value ^ 0.6) base XP for improving an item Source: http://www.uesp.net/wiki/Skyrim:Leveling
[old info] I did a little bit of experiment while leveling up smithing after 1.5 beta came out. Based on the data I have gathered, it appears that the experience gained is roughly proportional to the square root of the value of the item created. Furthermore, it also appears that experience gain by improving an item at a grindstone or workbench works in the same manner. The gain in experience is proportional to the square root of value added.
So it seems that improving expensive items to super legendary state will contribute a lot to leveling up smithing in later stages.
Data:
To get from smithing level 52 to 53, I needed to make:
To get from level 81 to 82, I made
To get from 81 to 82, I improved: