My understanding is that TXDOT is paying for the construction of the deck over the freeway. Landscaping, other items would need to be funded separately, but the deck is the big-ticket expense.
Edit: Page 5 of the Record of Decision references the cap construction:
“The Selected Alternative provides a structural “cap” over the proposed depressed lanes of I‐45 and US 59/I‐69 from approximately Commerce Street to Lamar Street. There would also be a structural cap over the depressed lanes of US 59/I‐69 between approximately Main Street and Fannin Street, and in the area of the Caroline Street/Wheeler Street intersection. Future use of the structural cap areas for another purpose would require additional development and funding by entities other than TxDOT.”
http://www.ih45northandmore.com/docs14/NHHIP_RecordofDecision/NHHIP Record of Decision - Body.pdf