Like I said, depends on how you negotiate. There's been a lot of consolidation in the industry, so there is less competition than before, and rates can change anytime.
90% is ok if the commissions are dirt cheap (around 20-50 cents per 1,000 plus/minus ECN rebates).
95% to 99% is much better, but you'll pay around $2 to $3 per 1,000 (plus/minus ECN rebates).
Remember there are fixed costs (data/platform) and variable costs (commissions, ECN fees, etc.)
If you're constantly taking liquidity (buying at the offer, selling at the bid), then a rule of thumb is to add around .002/share to .003/share to whatever rate you're quoted, as that will be the "true" rate you'll end up paying.
If you learn about the various routes and exchanges that allow you to add liquidity, then the opposite is true, and you can really offset your commissions over time (you'll pay some SEC/TAF fees, which are minor).