I've had two different home theaters; one with with a screen that cost about $50 and one with a screen that cost, well, much more than that. Here is a high-level overview of four options for a fixed, wall-mounted projector screen for any budget. Just add a front projector and you're ready for a real home theater experience.
Option 1: A Black Frame on a Painted Wall
You could just point your projector at a blank wall, but having a border makes a huge difference and doesn't cost all that much. This is what I did with my first home theater. The frame is simply 4 pieces of 3-inch wide baseboard molding, covered in black velvetine, held-together with 4 right-angle brackets, and hung on the wall. Please note that the black velvetine essential; see the second picture below for proof. There is a board in the middle of the screen in the second picture; there is velvetine draped over the very top and below it is painted flat black. Notice all of the reflected light from the flat black paint?
To make the faux-screen look complete, I painted the wall with a slightly gray eggshell paint. The paint was nothing special, just a white base with a small amount of black pigment added to produce a light-gray paint color. (The Gray helps blacks look blacker on the screen, which was more of a problem a few years ago than it is with the newest projectors.)
Pros: Cheap ($50 total), relatively easy to assembly, and no one ever realized it wasn't a real screen. You can build it to any custom dimension.
Cons: It was hard to get/keep the frame perfectly square. Imperfections in the wall finish can show-up in bright scenes. The screen screen gain (brightness) is very low; I'm not sure, but it was probably about 0.70 (vs. 1.0 to 1.5 for most commercial fabric screens), which makes the image noticeably more dim than a "real" screen would. The color accuracy may be less than ideal.

Option 2: A Black Frame on a Screen Goo Painted Wall
This is essentially the same as option 1, but with a different screen (wall) paint. The paint is called Screen Goo and it's available from Goo Systems. The main advantage is that it has a higher gain and better color accuracy than standard wall paint. Figure on spending about $200 for the base coat + top coat for a home theater screen (1000 mL of each.)
Pros: Improved gain and color accuracy vs. standard wall paint.
Cons: According to Projector Central, the gains for Screen Goo aren't has high as they are claimed to be. The Cinema White provided a 1.0 gain (not 1.8) and the digital gray provided a 0.75 gain (not 1.4). The imperfections in the wall and the problems with with squareness of the frame still apply as in Option 1.
Option 3: Take screen material from a low-cost screen and put it in your own frame
You can purchase a great screen fabric and install it in your own frame. Surprisingly, it's actually cheaper to purchase a manual pull-down retractable screen (you know, the type that goes over a chalkboard) and cut the material from it than it is to buy the fabric alone. People have been doing this with 4:3 (1.33:1 aspect ratio) Da-Lite screens ordered from AVSforum for years. Next, you just cut the screen to size and staple the fabric to the 3" fabric frame mentioned in Options 1 and 2. Total Cost is about $550.
Pros: Same image quality as a "real" screen.
Cons: Same problems as mentioned above with the frame, plus the fabric can have ripples due to the frame not being completely sturdy. Requires quite a bit of work. Requires destroying a new retractable screen to make your new fixed screen, which is a big step to take.
Option 4: Purchase a Complete Screen: Material and Fixed Frame
There are many different screen materials, many different screen manufacturers, and many different price ranges. You can pick a lower-cost manufacturer and get outstanding results (Da-Lite, Carada) or spend more and perhaps get slightly better performance (Stewart.) I think the lower-priced manufacturers offer the best bang-for-the-buck. Just pick a wide, black, fabric-covered frame and either a white screen (for best color accuracy and contrast ratio) or a gray screen (for better black levels.) Price: $700-$2000.
Pros: Perfectly, smooth and flat image. Great color reproduction and contrast (compared to other options; varies somewhat by manufacturer.) Very easy to assemble the frame, snap-on the material, and hang the included brackets on the wall.
Cons: There are so many manufacturers and screen materials from which to choose. Price is slightly more expensive than do-it-yourself options. You generally have to order the screen and wait a few days for it to be delivered (no instant gratification.)

Notes:
- You'll want the frame to be the same aspect ratio as your projector, which will most likely be 16:9 (1.78:1; width = 1.78 * height)