Cheap and home automation do not belong in the same discussion.
First you have the cost of the hardware. Is all of your equipment controllable (and just because it has a dedicated app doesn't mean that a centralized automation system will be able to control it)?
Then you have the cost of the automation system. Going DIY limits your choices, but allows the end user to control and modify their system instead of having a service tech come out ($$$) to make a change in the system. You've gotten a couple of suggestions already. I personally use Charmed Quark Controller (otherwise known as CQC) as the core of my system. I like their pricing structure much better than some of the alternatives. With CQC, you pay for a license based on the total number of devices you want to control. It doesn't matter what the devices are, and you can change the devices at any time. With most other options, you get charged for each specific device. If you end up changing brands (ie going from a Sony TV to a LG TV), then you have to pay more money to control the new device. With CQC, you can simply unload the old device and load in a new device without having to pay more money because the total number of devices hasn't changed.
Then you have installation costs. If you are going DIY, this may not require a lot of $$$, but it will certainly require a lot of time. The Professionally installed systems certainly cost more, but they are done in a stated time frame without a lot of grief. DIYing an automation system has been fun, but I'm a geek and enjoyed it. Unless you enjoy tinkering (and getting yelled at from the family because something went wrong), you shouldn't attempt to DIY this.
CQC user since 2005
SageTV user since 2007
PBX in a Flash user since 2012