It all depends on how you want to invest if day trading i say $20 bucks to get a feel for the market and experience the up and down spikes.
There is no minimum amount or maximum amount, you have to be very picky of investment advises, always invest what you can afford.
I think the start-up price is secondary, depending on your budget and your ability to control risk.
you need to start with 10% or 5% or even less of your funds which equal nearly 550$ / 275$ trade and investing to learn
Agree in risk control playing a very important roll as well,that been said there's no 100%earning in investment ,there's only earning lesser or losing all.Personally i choose 500usd cause it be more flexible for trading,at least i have extra capital to earn back if i loose