I actually don't exactly know what is benefit of 'prime broker' account and how much they may ask for depositing, or deposit is the only requirements for opening such an account or your need licence or recommendation letter from a professional body or referral by them,... to access the true ECN/STP by this way.
BUT, there is some brokers that may be more fair than others and you may search and find them(I couldn't find the best one myself, so I just working with some brokers by crossing fingers that they hopefully playing fairly). Actually I'm making money but there is a chance that they may waiting for your account reach certain millions the they play unfair and dodge you.
But surprisingly I seen some guys that playing spread betting in UK that is 100% market maker program, but they are making money (they bought new house and car).
I recently seen a broker that is offering fixed spread and they are market makers but some people making money from them too.
Update us is you got new info