Pond construction season is whenever the ground isn't frozen. If you can dig-it's perfect time.
Price for construction will depend on what company you use, size etc.
Ballpark, figure on in the neighborhood of $2000 for pond supplies (not rock material).
That is why in retail terms these things sell for so much. A typical backyard water garden will have $2000 in materials, $2000 in stone, & $2500 in labor.