What are the components? That will help us give you an idea for a good case for the components. For example, if you wanted to cram a quad-core, 8800GTX, four gigs of memory and two RAIDed Raptors into a mATX case, the smallest cheapest case will probably not sustain system stability.
Additionally, I do not like proprietary power supplies. Many mATX cases ship with their own dinky, low-quality power supplies that can't be replaced because they are built to a certain non-standard spec. If I'm building a mATX rig, I will pick a case that does not ship with an included PSU, and moreover, a case that fits standard PSU's.
With that being said, here's a small, inexpensive mATX case:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119149
Forty bucks. For that price, you can choose your own quality PSU, you get two external 5.25 and 3.5 drives and one internal 3.5. You also get a space for a 120mm intake and 120mm exhaust fans (which you will have to purchase separately, thus adding to the overall cost). It's a nice little rig.
For not much more, there's this one, also from CoolerMaster:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119088
Fifty bucks. Reverse ATX layout (similar to BTX layout). You can add a 120mm intake and two 80mm exhaust. Ifyou buy a PSU with a 120mm fan that draws from the underside, then you can use your PSU to help vent the hot air directly coming off an 8800 series video card (or any other video card, for that matter). I have one of these that I am in the process of modifying for a watercooled mATX build.
Cheap is good, but so is quality. Here are a few examples of
very nice mATX cases from a few different manufacturers. They offer different cooling capacity, but they also come in different sizes, and they all are compliant with standard ATX power supplies.
Here's a Silverstone. This one's also available in silver.
http://www.newegg.com/Product/Product.aspx?Item=N82E16811163085
Another Silverstone.
http://www.newegg.com/Product/Product.aspx?Item=N82E16811163058
An Antec.
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129041
A slim, sexy Lian Li (!), also available in silver.
http://www.newegg.com/Product/Product.aspx?Item=N82E16811112143
Partial to cube cases?
An NZXT, also available with red lighting.
http://www.newegg.com/Product/Product.aspx?Item=N82E16811146043
Here's one from Raidmax. Notice the watercooling grommets in the back.
http://www.newegg.com/Product/Product.aspx?Item=N82E16811156200
And here's another Lian Li, again available in silver.
http://www.newegg.com/Product/Product.aspx?Item=N82E16811112151
Edit: Not one of these cases comes close to your criteria of "smallest, cheapest mATX case". Sorry. I was never good at coloring inside the lines.