It was this before 1.1.0 I think. After that it's like Skjuld mentioned, defined by damage and also weapon speed. It's capped at around 60k to sell. This cap raises as your trading skill progresses.I was assuming that the difficulty of the weapon dictated the experience... Yet another system where people have to meta game it for it to work..
--snip--
I raised my smithing skill easily from ~170 that I had before that patch to 320+ by mass crafting a type of tier 4 polearm that did 152 damage at 81 speed.
Only needing 2 steel and 1 hardwood for around 19k base exp.