Disclaimer: while I have not built any tables the ways that I am describing, I have built tables more complex than this, and these are rather my general suggestions based on my experiences with more complex tables.
Recommendation: 4'x8' sheet of plywood
Approximate cost: $12.50 (per board)
Recommendation: Wood glue and sawdust
Approximate cost: $7.50 (for 16 oz. of wood glue)
Recommendation: 2 sample sizes of interior latex paint, flat finish
Approximate cost: $6 ($3 each)
4) FlockingThis is the often overlooked part of putting together a good board but can really make a huge different. It can even make up for spotty paintjobs or other miscellaneous errors during the rest of the process. You have a lot of options here: Games Workshop, Army Painter, Gale Force Nine, and Woodland Scenics. You'll definitely need to buy flocking for the whole table, so it's best to find the best price for the volume, so I'd recommend Woodland Scenics for flocking, where you can get a bag of 54 cubic inches of flocking for only about $8 - which is about as much as a small tub is for Games Workshop flocking!
Recommendation: Woodland Scenics turf
Approximate cost: $8
Instead, I recommend purchasing the Gale Force Nine Battlefield-in-a-Box terrain kits as they are often a reasonable value and are already painted for you. I own a couple of these kits and I cannot recommend them enough for those who don't have a lot of time. The only problem? These things are so awesome that they are currently sold out! Also, since we're trying to build a table cheaply, let's see how we can save money the most. Well, if you're really looking to cut corners, I have looked into fish aquarium terrain, as it's often fairly cheap, is pre-painted, and can offer some great visual variety. If you want forests or water features, you can also just head over to the fabric store to pick up some green and blue felt to cut out into the shapes that you want. Warmachine/Hordes doesn't worry so much about the physical objects in the terrain - only the area of coverage!
Recommendation: Aquarium terrain + felt
Approximate cost: $50 will get a good selection of terrain and felt is really cheap.
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Okay so let's put it all together and see how much this will all cost us.
$7.50 (wood glue and sawdust)
Total: $84, although without terrain it only costs us $34 to make most of the table! That's not shabby at all! By saving money on our supplies and construction methods, that leaves us with lots more cash to spend on terrain, and if you budget about $100 for a table (very reasonable), then you can give yourself ~$60 to spend on some nice terrain for your board - although terrain is something that can be collected over time and expanded (just like an army).
I hope this inspires everyone to go out and put together their own table because, as you can see, it's not as expensive as you might expect!