It will run you about $4.00 each way, there is a toll booth just after you get onto the Turnpike when going south and I think it is $2.50 and then there is another small charge when you get off at Osceola, and then the same costs going back North. Hope this has helped.
According to the toll calculator on the Florida Turnpike website the cost is $2.50 for the turnpike from I-75 to Osceola Parkway. Remember that the Osceola Parkway has a $1.25 toll to get to WDW.