Pegasus Hobbies have modular plastic Gothic Building kits. There's a main wall/door kit which gives you, I think, 24 x 10cm/4" sections - certainly enough to fill a 2x2 board for £25/$40. There are also a couple of add-on packs that include buttresses, rose windows and pitched-roof gable ends for a lower price.
None of these packs have floors included, but they do have supports so you can add your own between the walls.
There's also a set of ruins that can stand alone or connect to the other sets - you get 2 corner ruins that are 20cm x 20cm (8" x 8") and include the first floor ruined corner (2nd floor for some of you).
Puppetswar have just released a series of ruined buildings, ranging from a small ruined chapel all the way up to a ruined cathedral. Again, that cathedral would fill a 2x2 board, for IIRC €110. Resin I'm guessing.
Games Workshop sold a very nice Osgiliath Ruins kit for their LotR line, which was a single ruined building in three sections for about £20. It's now OOP, so snap it up if you see one!
The Dust Tactics building was more modern and blocky than the Pegasus buildings, but was also modular, plastic and very good value.
Carving your own foam walls is a good way to go, and Hirst Arts moulds do well for both walls and some of the embellishments like columns, windows and domes. Plastic columns for supporting wedding cakes also good!
Edit: Just seen folks mention the GW LotR ruins.