Thanks to Solomid, there is an item efficiency chart so you can check which items are really worth buying.
This database is a couple weeks old but still shows everything before the new item and the manamune. It gives you efficiency scores along with notes. Also it shows how efficient a snowball item is when its first bought, broke even, and full.
A great tool for knowing which items you should buy, and which items you definitely shouldn't buy to maximize your item:gold ratio :D
Item Efficiency Spreadsheet
Thanks Dret from Solomid!