I've suggested this for Sci-fi terrain before, but it's just as good here I guess...!
Step 1: Get some cheap nested gift boxes. Something like
these perhaps, but you may have a local dollar store or such that sells 'em cheaply (Ikea sometimes does for example). Be aware that they even come in different shapes too, so that could be an easy way to add variety.
Step 2: Cut windows and doors into them. Some of them you may want to cut into more to represent damage, or in half to make a ruined corner.
Step 3: The lids make good low-level ruins or can be used on more intact buildings to represent roofs/floors.
Step 4: Add some details like buttressing, lintels, cills, etc with some strips of card. I'd keep these fairly flat (I'll explain why in a sec).
Step 5: Spray the whole lot in cheap grey primer. You can use other colour primers too, both for some variety, and to build up a gradient of colour on the buildings quickly.
Step 6: Once dry, you can stack all these back together for easy storage and transport. (That's why appliqué details are best kept fairly flat).
If you have a stencil or a stamp (a sharp knife and a cork or vinyl floor tile will let you make one), you can use this with some cheap paint to add some blockwork details to selected areas of the ruins. Likewise, you can add scraps of suitable textured wallpaper samples (these are usually free) or plasticard to add more texture and interest.
Some of the nested boxes sets come with very small boxes in the centre - these can be dressed up to make chests/crates for cover, or might make little guard posts or such if not.
A few packs of boxes like these will easily fill a table.
As for other things... Well, you can probably find a lot of what you need at home:
- Scrunch up some paper, cover it in a couple of layers of aluminium foil (scrunch this lightly around the paper core) and glue to a base. Spray grey, wash with thinned black and drybrush light grey - instant lightweight rocks or boulders in any size you need.
- Rough ground can be anything from pieces of fabric to textured wallpaper scraps. Just paint these quickly and drop a few around the table to represent areas of difficult terrain. They also look good with a couple of trees parked on top for area terrain (where the trees are moveable to let you place the figures "inside").
- Christmas/Halloween decorations and wedding favours are also a good source of things like trees, crystals, figurines (to paint as statues), etc. Dollar/Pound/Euro stores too; they seem to sell this
tat valuable treasure all year-round.
- Whilst you're in the Dollar/Pound/Euro store, look out for cheap kids' toys that you can appropriate for set-dressing. Dinosaur skeletons to partially buy into a base (the bones of a perished magical monster in Frostgrave), playsets that may be re-purposed into lairs or temples, figures than can make statues, pirate sets with crates and barrels that you can pinch, etc.
Obviously, if you spend time sourcing, making, and re-painting stuff, then it will look proportionally better. However even the basic stuff (like the boxes) can look great in a pinch.