The previously announced Titanfall K'NEX playsets are coming this fall. There will be six different sets, ranging from $13 to $45.
The priciest ones include over 500 pieces and will let you build out Angel City. $35 will let you build a 12" Titan, which includes a pilot figure that works within the mech. You can see all the purchasing options at Kotaku.
Here's a quick 360 degree view on what a K'NEX Titan looks like: