Gov. Phil Murphy fulfilled one of his campaign promises when he signed legislation on Wednesday making New Jersey the 10th state in the nation to offer to help pay college tuition for immigrants living in the country illegally.

The law makes unauthorized immigrants eligible for the same state-funded college financial aid programs available to other New Jersey college students -- including the Tuition Aid Grant program considered one of the most generous in the nation.

How much will this cost New Jersey taxpayers?

Early estimates say the program will cost the state about $5 million in its first year.

But, even supporters of the new law say the potential final bill of new financial aid program for unauthorized immigrants is unclear. State officials are unsure exactly how many New Jersey college students are living in the country illegally or how many might apply for state financial aid.

"We don't really have an estimate," said Jennifer Azzarano, a spokeswoman for the Higher Education Student Assistance Authority, the state agency that administers college financial aid programs.

Students have until September to apply for financial aid for the fall semester and it remains to be seen how many of "New Jersey's Dreamers" will be eligible for state grants and scholarships, Azzarano said.

Unauthorized immigrants are still barred from applying for federal financial aid, including Pell Grants and federal student loan programs.

Some lawmakers at Wednesday's celebratory bill signing at Rutgers-Newark estimated the cost of opening up state financial aid programs to unauthorized immigrants will be relatively small.

Assemblyman Gary Schaer (D-Passaic), one of the sponsors of the legislation, estimated about 600 unauthorized immigrants will join the more than 66,000 students currently receiving financial aid through the state's TAG program for low-income students.

"The total cost to the state of New Jersey for these Dreamers as part of the TAG program -- about $5 million. Ladies and Gentlemen, for each taxpayer in this state that will cost 17 cents," Schaer said.

However, those estimates are based on an analysis using 2015 data gathered by the state's Office of Legislative Services. Their researchers said it would be impossible to determine exactly how many unauthorized immigrants would apply for and qualify for state financial aid under the new law.

The office estimated it would cost about $4.47 million in additional taxpayer money to cover financial aid awards for unauthorized immigrant students for one year. That is about a 1 percent increase in the current state budget for TAG awards.

Researchers reached the estimate by taking the average TAG award for a student at a four-year college -- currently $7,451 per recipient this year-- and multiplying it by 600 students.

"While the OLS does not have sufficient information to determine the amount of financial assistance for which students would become eligible, existing data suggest that the fiscal impact would be small relative to current appropriations," their report said.

An NJ Advance Media analysis published earlier this week showed the number of unauthorized immigrants enrolled at New Jersey's 11 four-year public colleges is rising. There were at least 758 unauthorized immigrants enrolled this year, though the count did not include Rowan and William Paterson universities, which did not release the data.

It is unclear how many students at New Jersey's two-year county colleges are unauthorized immigrants who could now be eligible for financial aid.

It also remains to be seen if being eligible for state financial aid will prompt a surge in unauthorized immigrants applying to attend New Jersey colleges.

"College has always felt out of reach. Today, that changes," said Maria Del Cielo Mendez, a high school senior from Plainfield who spoke at the bill signing. She plans to take advantage of the new law to apply for the financial aid to attend one of the six local colleges where she was accepted.

Gov. Chris Christie signed legislation in 2013 that allowed immigrants living in the country illegally to pay in-state tuition at New Jersey's public colleges. The Republican governor rejected a plan to open financial aid programs to unauthorized immigrants.

Murphy's election helped revive the Democrat-backed legislation. However, Republican lawmakers continue to blast the new law, saying it is unfair to give financial aid to those living in the country illegally when many legal residents struggle to pay college tuition.

Unauthorized immigrant students have until Sept. 15 to apply for financial aid for the fall semester, state officials said.

Kelly Heyboer may be reached at kheyboer@njadvancemedia.com. Follow her on Twitter @KellyHeyboer. Find her at KellyHeyboerReporter on Facebook.