You can use almost anything as a 'digital transport' - even your 'phone with a suitable dock. However, everything else in the system is very sensitive to broad-spectrum interference - particularly the clock in the DAC. Timing errors and jitter produced by transports also result in characteristic differences between CD transports, computers and streamers.
So, yes: you can take a stock dock or computer, and it will play music through its headphone outputs, or via a soundcard or off-board DAC. The more serious you are about doing this properly, the more careful you need to be about the specification of the power supplies, shielding and vibration dampening, and the more choosy you have to be about clocking and everything in the local playback environment.
As the poster above notes, you can start with a basic computer and apply some common sense modifications, or follow best-practice 'recipes' such as those found at Computer Audiophile, or here:
http://www.itemaudio.co.uk/media_server_pc.html
As you start to specify higher quality components (particularly power supplies), the cost starts to rise: the CAPS machine linked above will cost you more than £700 to build using a bog-basic PSU: £1000-1500 if you want to power it cleanly.
It's perhaps easier to see now why the Aurender, which was developed from the ground up using mainly proprietary parts (not an off-the-shelf, mass-produced motherboard with its many, many problems for audio use) - and multiple bespoke power supplies - could rack up production costs.
It's exactly like buying a new car: you can get a perfectly serviceable runabout for £8-10K. You can get a very nice SUV that will do a bit of everything very well at double the speed limit for £35-40K. You can get a great used family car for less than £5K.
But a Lamborghini, which is full of massively 'over-engineered' hand-made parts, and doesn't carry more than a bag of golf clubs, or more than two people, and is as uncomfortable as heck - and will be ruined by speed bumps - costs £150K+. A Lamborghini is designed to do just one thing very, very well. The Aurender is like a Lamborghini in that regard.