I think USD 100 will do, presumed that the prices are already stable and the market is healthy, free from hysterical and overbought. Trading with scalping technique by spreading your money in at least 5 prospective coins is recommended, with the biggest portion is in the btc.
100 Dollars is quite a small amount for trading especially for the scalping purpose. Scalping should be done with big amounts because it wont give you much profit if you do scalping with small amounts. Traders with small amount of initial capital should invest in a coin and then wait for atleast 10-20% gain.