Lux is my main champion and i probably have over 500 games played with her in my experience this is the most cost and damage efficient way to build her ( with upcoming S3 this will change and Riot has said that they are doing itemization changes at the start of the season).
For mid lane:
1. Boots x3 hp pots
2. chalice of harmony (give you infinite mana in lane and gives you the edge on your opponent cuz of the built in MR)
Upgrade boots into Either sorc boots if theyre building MR or GET the CDR boots.
3. Now you have a choice depending on how your game is going you can either get Kages lucky pick (do this if your losing out on cs and need the boost in gold) OR finish your chalice into the grail.
4. Rabadons DC (nuff said)
5. and 6. Rylais( OR if you went the kages rout get your DFG)
7. Last you can get a lich bane and make kids cry (MR heavy team sorry youre gonna need a void staff and shoot lasers of death)
Hope you like it!