If you're not too heavy, the titanium bottom bracket offers the most grams-per-dollar savings. These days, with light wheelsets in mail-order abundance, there's a lot to be saved there, too.

Rough numbers to remember:

28 grams to the ounce
16 ounces to the pound.

Now, go save 100 grams on tires and another 100 grams on yor bottom bracket and look what you done!