When you go to Megaton you can go to the Brass Lantern and wait until nobody can see you.

Then hack their terminal - it will say unlock floor safe. Do that and open it - you get 300 caps.

Colin Moriarty will want you to talk to Silver - get the caps from her and by now you should have 700 caps.

If you dont want to spend 100 caps, get Moriarty's terminal password and see where your Dad went.

After that talk to Mister Burke, pass the speech check and get 1000 caps from that (will give you bad karma).

You will then have 1700 caps.