My philosophy has been to upgrade components individually (apart from new CPU with motherboard and RAM) over many years. So I will generally keep a case, GPU and PSUs over a number of years. Now I can get a better idea of the total actual value of all the components as my current built is nearly identical to the example in the linked article.
The example given MyBroadband for a pre-built PC is interesting as it is not an off-the-shelf standard computer but you can specify a number of components yourself which is quite important for gaming. Every component for gaming is important right down to the case, fans and PSU.
See Why you should consider a pre-built gaming PC
You can get great savings buying a pre-built PC when compared with buying the components yourself.