A flat fee broker keeps a flat fee, rather than a percentage, of each of your commission checks.
This is quite uncommon among brokerages.
Usually a brokerage will charge you a significant percentage of the overall commission - 25% on the lower end and up to 50% on the higher end. In addition to this, the broker will often have monthly fees they charge for office, equipment usage, supplies, etc.
Imagine you make $10,000 in commission on a deal. If the broker takes 25-50% ($2,500-$5,000), that’s a huge chunk of your earnings. Add to that any other monthly fees they charge.
On the other hand, if you find a flat fee broker, they may charge you $500 per transaction regardless of what you make. In the example above, if you made $10,000, they’d take $500 and you’d make $9,500! Wow! Quite a difference.
In this scenario you’d have to do between 2-4x...