I started trading my account with $ 1,800 trading mini lots back in 2004. I think anything around this area is probably fine even today with all the changes that brokers have had with leverage and stuff. Of course the more balance you have the better.
In my opinion $1,500 is the least amount of money I think anyone should put in to trade a mini account.
Since Micro accounts have appeared, I think $500 should be enough. Trading is like Poker you know, in poker you want to have at least k blinds in chips to have room to play under ideal circumstances (depending on cash or tournament), all in all it’s the same reasoning for Forex. Bottom line is you want to have room to maneuver through a draw down.
Edit: forgot to mention, you can do the math, but it’s not hard to figure that I think around $15k-$20k is a minimum balance for a standard 100k account.