Keep in mind that if you're removing liquidity on most trades by selling at the bid or buying at the offer, then as a general rule, you can expect add another $2 to $3 per 1,000 on that rate in commissions.
That will put your true commissions at around $5 to $10 per 1,000. So for example, a new trader who does not add liquidity and does around 100k shares per month will pay around $500 to $1,000 in commissions alone!
That does not include platform fees and professional data fees.
Before you join a prop firm, it's best to evaluate an estimation of your true monthly costs, both fixed and variable, so you can define the amount of money you'll need to make on a monthly basis to justify the costs.
If you can add liquidity to most of your trades, then you can greatly reduce your variable costs per month, and perhaps even get them close to zero. Those who ignore their variable costs will end up as "commission churners and burners" which is great for the firm, but not so great for the trader.