two factors to consider when building a matrix, 1) viewing distance and 2) content resolution. you have two methods to use based on this. If your viewing distance is 100ft+ then you might look at the pixel "nets/panels" that are currently out there. I have used the Boscoyo products with great success. Only down side I see is pixel density, they are 1 inch spacing and depending on the size you have to push the pixels. First ten pixels are fun to push but when you get to pixel 10,000 your thumbs might hurt a bit. Because of the one inch spacing there are limits on the resolution of the content you send (might be ok with cartoons and standard definition).
P5/P10 (the 5 and 10 signify the millimetres distance from one pixel to the next) panels offer another option if your viewing distance is less than 100ft. They are purchased by panel and start at $11 for one 6" x 12" panel. The beauty of this product is that you can clip 96 of these together and they can be controlled with a Raspberry Pi (costing $35) and running FPP) and a color light board ($25). P10 panels can handle 720p content and P5 panels can handle 1080p.
Panels are cheaper than bullets and IMO easier to setup config. One strand of bullets (50 pixels) is around $19 of 38 cents a pixel and you have to push into a panel. One P10 panel is $11 and has 512 pixels (at a cost of around three cents per pixel), either plug into your colorlight board or into the next board depending on your configuration. One P5 panel is around $17 and has 2,048 pixels in them for a cost of around a penny per pixel.
I currently run two P10's and a P5. They work great and are easy to setup.
Only issue with any matrix IMO is how and where to mount, weather and your location can be a factor.