So are you planning just to have it Halloween night? Anyway here are some relatively inexpensive ideas....
First of all buy some black plastic from your local Ace Hardware or Home Depot... They come in a bunch of different sizes and thicknesses but if this is for your garage then you probably don't need some heavy duty expensive stuff. You can probably get a roll big enough for about $15 so that shouldn't be a problem. While you're there look out for burlap or jute netting... this really comes in handy while adding detail and distress to your walls however both are pretty expensive.... probably $25 for a small roll of burlap and anywhere from $50 - $100 + depending on the size of the roll of jute. For your purposes go with the burlap, tear it apart, add a bunch of holes in it and then drape it along your walls. You can also go to a fabric store and try to find something cheap that you can tear apart for the same effect, although I haven't gone yet so I don't know what the prices are or if theres anything worth buying. What props or costumes do you have by the way? You're definetely going to want a bunch of actors because pneumatics are pretty complicated especially if this is just a small haunt I wouldn't bother with them. Alright on to props that you can build yourself... if you don't have a big budget I would suggest going with Bluckys... the blow molded skeletons found at big lots, party city and most halloween stores. See my haunt thread....
http://www.hauntforum.com/showthread.php?t=6978 for info on how to make these guys look pretty good for the price. Trust me its not complicated at all... if you want to make them then PM me so I can give you some more tips.
Now this is a VERY cool effect that we built last year over the weekend before our haunt and although it might cost a bit it is definetely worth it in the end. It's called the living torso illusion and it will leave a huge impact on your guests. It will probably cost anywhere from $70 to $100 + depending on how big you want the table to be and what price you get for the mirrors. Again if you're interested just PM me.
Now there's also a bunch of really simple things that you can do to startle people. Buy a cheap strobe at party city for 10 bucks and attach it to the ceiling so that it points towards the guests. By the time they walk up to it they will be completely disoriented which is when an actor jumps out for the scare. Also on some of your walls get some flourescent paint and paint a simple pattern on it (it could just be random lines, it really doesn't matter too much) then if you have some old black clothes or some cheap halloween robe, paint the same pattern on to it so that the actor blends in to the wall and then pops out. Honestly, if you want to have a haunt the only thing you need are actors and black sheeting, but if you want to have a great one then you're going to want to add some extra detail or simple yet cool effects. Hope it turns out great and if you have more questions feel free to PM me