How much should it cost to convert a covered porch into a 3 season room?

We have a 12 x 15 porch on a concrete slab outside our kitchen. There are two posts on the outer corner that support a wooden pergola type covering over the top. Approximately how much should it cost to enclose this porch as a 3 season room?

It would cost approximately $12-15K to install a panel system with interchangeable glass panels and screens, with a roof framed over. It would include architectural shingles. There is some unknown conditions as to the existing roof structure being tied into, and the condition of the existing roofing on the main building. To provide an accurate price I would need more information