I think there is no minimal invest, you can start invest to trading only for $20 or less, it all depends on where exchange you want to trade.

Because each exchanger has a different minimum deposit requirement.

May i suggess ? If you want for long investment you can go to bitcoin. But if you want for short term investment then go to altcoin.

Just make sure you already know what is the risk of trading bitcoin or altcoin. And keep your safetybelt and dont be greedy buddy.