I wonder why nobody has mentioned terrain options from Dropfleet Commander yet (or did I miss it?). The terrain is scaled at 10mm, so might be a bit on the "too big" side - on the other hand that makes it easier to get actual cover even for larger titans.
I consider the
Ruinscape Card Scenery Pack to be the best compromise between price, looks and playability. It gives you a generic city board with "battle damage" in colour and easy to set up right out of the box, can cover up to 6'x4' table space and offers a huge variety of setup options - all for less than a squad of Space Marines.
It might not be for everybody, but it is very affordable and incredigly easy to use.