They are not cheap after you factor in costs for installation, the framework, land, etc. Increased efficiency means you also need less of all those.
there's a company now that's laying them flat reducing the cost significantly.
Removes the cost of the frame, reduces efficiency. Presumably whether it's a net gain depends upon the site, eg terrain shape, latitude, need for passive cooling, wind-blown dirt, local fauna, etc.
Can we make pedestrian paths using solar panels yet?