AGPO:
I’ve recently started work on a new 40K army (yes, I know) and want to give them rugged, industrial themed bases. The army is going to be a small elite space marine force with quite a few large bases, so I’d like to really go to town on them. Despite this, I don’t want to shell out for resin bases (I prefer to do my own work and have unique bases) or GWs extortionate basing kits. Can anyone advise?
nitroglysarine:
industrial, hmmm have you got an old computer/electronic device?
If so rip out electronic components and use them on the bases, they’ll create a cool effect.
resistors, leds, capacitors and the like.
Thorne:
Not to mention gluing mosquito netting glued to bases then painted black and drybrushed boltgun metal looks rather cool. Making razor wire from bonsai or gardening flourestry wire. Rubble can be made using excess filler left to dry overnight and smashed into useable sized peices.
Tarrakk Blackhand:
Get old, broken watches and glue the little gears, springs and so on to the base. Also, try and get a hold of aluminum shavings from a factory or some such place. The cruder the shavings, the better.
nitroglysarine:
@Throne, nice idea about using the filler then smashing, not tried that!
@Tarrakk, eBay has loads of them, but you’ll have to compete for them with Cosplay and Steampunk Larpers!
G.2:
I have been collecting old and broken printers for the purpose of salvaging their insides for basing and modelling. You can usually find them on the side of the road. 
Not that you would ever see me stop and picking them up.
The moulded plastic cases usually have ridges and groves on the inside, so you can use ‘cut out’ pices of printer cases, which can be stuck directly onto your bases.
Note: Save all the little ‘stainless steel’ screws from inside the printers as paint agitators - saves money on buying ball-bearings.
GRNDL:
Use plasticard to make spaceship decking. Its cheap and easy to create variations. Cut out a rough circle to cover the base, use a dremel/sanding block to rough up the edges, scribe panel lines, etc. Double up on the thickness for variation. You can use screen door mesh to make vents. If you can get some extruded styrene hex and round rods, you can make bolts: slice of hex + slice of round on top.
For painting, you can get a wide variety of styles depending on how you finish it. Straight metal finish = nice and clean (Imperial spaceship), but by stippling browns +orange reds then drybrushing metallics, you can get a nice disused space hulk feel. Paint bits of it in red and you get an orkish spaceship feel.
I’ve been using this sort of style base for my Ork Pirates and my Space Skeleton submission for the Brush Slave comp. Its fairly fast and gives a good results without tons of attention or care.
Loki:
for rubble and the like , just cut the sprues that the models come on to about the size of bricks and pile them on the base in little heaps with some wires sticking out or maybe an arm here and there , also a little bit of brass rod cut up small make great shell casings
Thorne:
The best ever shell casings I tend to use .22 long percussion caps for american civil war reinactment. Sometimes I have 9mm shells laying about that dontget wasted for bigger artillary shells.