If you are on a tight budget and don't care about upgrading your desktop computer system, the Acer Aspire AXC-603-UR12 can be found for as little as $300. The system will provide enough performance for those that just want something to browse the web, stream media and do productivity applications like Word or Excel. It is a compact desktop system that looks like it might have room for some internal upgrades but it really does not beyond the RAM. It uses an unusual Pentium J2900 quad core processor that gives it a decent level of performance but it is held back by the 4GB of memory and 500GB hard drive. Still, it is tough to find a less expensive desktop system.
For those that have a budget but don't want to have to sacrifice features, the Dell Inspiron 3000 Small is a surprisingly well equipped slim desktop system. It features a Intel Core i3-4150 dual core processor that gives it a performance boot over most at this price point. While it still just uses 4GB of memory it offers a one terabyte hard drive that is twice as much as most systems in this price range. It even offers wireless networking which has been dropped by many companies to keep the costs down. About the only downside is the slim case makes it a bit more difficult to add in a graphics card for those that want a low cost system that can be used for PC gaming.
One of the major reasons for getting a low cost desktop system is to integrate it into a home theater system for browsing the web, streaming media and checking mail. A Windows PC can be a bit of overkill for this task such that a lower cost alternative like the ASUS Chromebox might be an option. Rather than Windows, it uses the ChromeOS the is specialized for internet use in the tasks mentioned above. It won't run Windows programs but if you do most of your tasks in a browser then this is very affordable at just $180 for the basic model and $400 for a version that can support 4K TVs and has more RAM. It just have just 16GB of storage though which could be a problem for some but at least it has four USB 3.0 ports which is more than most desktops. You will also have to supply a mouse and keyboard with the $180 version.
With so few low cost desktops on the market, another option for consumers is to build their own desktop computer system. $400 is a very tight budget but it can be done with the right parts. I generally recommend people move up to $500 as it provides a more rounded system. It actually is not that difficult to do and I even offer a 5-day newsletter course on how to put it all together. The tough part is finding which parts to use in your build. Thankfully, I even offer a guide with suggested parts that can be found to put together such a system that can always be modified down to reduce the final price.