Comcast has unveiled a new $30 charge that will let customers in Florida escape the company's 300GB monthly usage limit.

The nation's largest cable company has been trialling data caps in nine states, with slightly different policies in each one. Generally, customers who exceed a monthly limit pay an extra $10 for each additional 50GB, though customers are allowed to exceed the caps for three months before getting penalized.

But customers in Fort Lauderdale, the Keys, and Miami, Florida, can now purchase unlimited data for an extra $30 per month. Paying this additional $30 eliminates the 300GB monthly cap, but customers have to pay the extra amount each month even if they use less than 300GB.

"The Unlimited Data Option costs the current additional fee of $30 per calendar month, regardless of actual data usage," Comcast said in an FAQ updated today. (Thanks to DSLReports for noticing the change.)

Customers who use more than 450GB per month may come out ahead by purchasing the unlimited data option.

"For example, if you enroll in the Unlimited Data Option and use 530GB in a given month, you will only be charged $30 for choosing to enroll in the Unlimited Data Option," Comcast says. "If you do not enroll in the Unlimited Data Option, you would be on the 300GB plan and therefore would be charged $50 for the additional 250GB (five blocks of additional 50GB) provided on top of the 300GB plan. Note that customers enrolled in the Unlimited Data Option who use less than 300GB in a given month will still be charged $30 for that month."

The unlimited data option hasn't been made available to the other eight states where Comcast is imposing usage limits. Those are Alabama, Arizona, Georgia, Kentucky, Maine, Mississippi, Tennessee, and South Carolina.

Within the trial areas, customers who buy the pricey 505Mbps or 2Gbps plans don't face data limits. Customers who live outside the data cap trial states don't face any limits or overage charges regardless of what plan they buy, but Comcast may impose limits throughout its territory within a few years.

If you're wondering how Comcast settled on a 300GB data cap, a company VP recently said that it's a business decision rather than one driven by technical necessity.

Customers who face the data cap told Comcast in a survey that they're interested in an unlimited option, even if it costs extra, the company says.

"In a customer survey with some heavy data users, 60 percent expressed interest in an unlimited data plan option at the predictable, flat price of around $30 a month," a Comcast spokesperson told Ars today. "So, we’re giving that a trial run in a few Florida markets starting on October 1st and we’re notifying customers there about a month in advance. What we’ve found is that about 10 percent of our customers consume almost half of all the data on our network, so these trials are based on principles of flexibility and fairness."