The amount of credit you receive ha a maximum. Once you pass these eligible amounts, adding more expenses doesn't help.
From the IRS:
"The amount of the credit is 100 percent of the first $2,000 of qualified education expenses you paid for each eligible student and 25 percent of the next $2,000 of qualified education expenses you paid for that student. But, if the credit pays your tax down to zero, you can have 40 percent of the remaining amount of the credit (up to $1,000) refunded to you"
$2500 of it is called a nonrefundable credit, meaning you can only use the amount up to where your tax owed reaches zero. After that, you can get up to $1000 in cash, but that's it."
