Assuming you don't mind shopping on AliExpress and EBay:

Carbon post: 100g, $20
Carbon seat: 150g, $20
Carbon bars: 100g, $30
Ti skewers: 50g, $10
Light pedals: 75g, $100
Lighter wheelset: 300g, $300
Race tires/tubes: 150g, $100
Carbon frame: 300g, $300-$600

Figure 450g per pound. Anyway, it's considerably easier to start with a light rig than to slim a fat one. I've done the first five things on this list to bring my flat-bar below 20 lbs. The big upgrades (drivetrain, frame, etc.) are considerably less expensive if bought as part of a complete bike.