The Key is once you get to level 105 then to buy as much tin as possible for less than 3s per bar/ore. Use the AH mostly.
Then go mine lots of copper and make bronze.
Once you have the bronze get the recipie for Rough Bronze Leggings. They take 6 bronze bars only.
Make only Leggings until you run out of bronze.
Sell the leggings to a vendor for 9 silver 64 copper.
It should only take about 140 tin bars/ore To reach 150 smithing
At 3s each that's 4g 20s, but from selling all of the leggings to vendors you can make 4g 43s 44c thats a 23s 44c profit.
It works very well to get your smithing up and you can tun large profits from getting tin at lower prices like 30s for a stack of tin bars.