There are really few passive projectors on the market so if you want to do it, then it means DIY dual-projectors. There's quite a lot of information to learn about it before being able to choose which hardware you'll buy, but with a little bit of work you should be able to do it.
The bad news : it's complicated, you'll have to set up and use many parts every time you want to use your dual-projectors, and if you want to use a wide variety of 3D compatible devices (aka : hdmi 1.4a) it will significantly increase the price.
The good news : if you don't aim too high in your picture quality requirements it isn't that expensive.
Finally since price is your main criteria there is one thing I want to be brutally clear on : unless you have a very large audience, for any given picture quality, an active projector will be significantly cheaper than a passive dual projector system. The only real big advantage you'll get from passive is (and it might be an important one) : zero flicker and simultaneous image presentation in both eyes. If you don't suffer from the issues caused by flicker or frame sequential presentation, then you should know you'll get a better deal for your money with an active projector.
I'll give you some brief ideas but not much detail since the exact makes and models you'll want to buy will depend greatly on your tastes for your environment, and which part of picture quality you'd like to prioritize over others (indeed, currently available passive technologies are not perfect)
For rock bottom prices :
-A pair of cheap 720p DLP projectors, No lens shift, No fancy features, but just make sure you get some with very high lumen output (2000 Lumen is an absolute minimum for a 100" screen). They do not need to be 3D-ready, just regular 2D models will do. If you have a little bit more budget, you can try the entry level 1080p ones, but they have to be DLP (you can also use LCD projectors but it will trigger more complexity and/or higher costs later)
-Omega optical filters + glasses kit
-A white wall as a screen (no silver-screen required, it's not polarised, but you can use a cheap white matte screen or a high gain screen if you want to)
-A very tightly controlled lighting in your room : The omega glasses are extremely sensitive to ambient light, you' will need a very dark room, if possible with dark walls around you (or dark furniture to cover as much white walls as possible) Ideally, the only white thing in the room should be the screen.
If there are any elements above you do not like, then you will have to consider many many more options such as polarisation technology (which requires a silverscreen), LCD projectors and finding which ones you can and which ones you cannot use according to your requirements
-As a video source, to keep prices low, you would use your current desktop computer (being compatible with hdmi1.4a devices like a BluRay 3D player or a Sony PS3 is possible will cost a lot since demultiplexers are very expensive at the moment, perhaps the situation will change later). In your desktop PC You'll need an ATi graphics card which has the Eyefinity feature and which has two identical outputs (2x DVI for example) (at least ATi Radeon HD5000 series or more recent), if you are not a gamer and just want to watch movies you can find a new Radeon HD5750 online for less than 80€. If you do play games then it's more complicated, but then you probably already know you're not on a budget because you'll first need the big gaming PC with a beefy CPU and the most powerful graphics card you can find, In any case, you'll need an ATi one for dual-projectors. The Nvidia 3D Vision stuff is not compatible with dual-projectors.
Some reading : this is a big summary about everything there is to know about dual projectors http://www.projectorreviews.com/projector-technical-blog/passive-3d-projection-part-1.html
The "big" dual-projectors discussion thread here at AVS http://www.avsforum.com/t/1280393/the-ultimate-3d-projection-system-a-practical-discussion-thread
The Omega optical thread : the glasses ans filters technology I recommended above DO READ IT !!! it does have some constraints you must be aware of before buying. http://www.avsforum.com/t/1407101/official-omega-3d-passive-projection-system-thread
Additional informations : read the thread in my sig, it's about the dual-projectors I have at home. It's polarised, and some of the info is a bit old (I haven't updated it for a while) but I think it's probably a good read.