If you're looking to save money by building it yourself forget it because you won't.
Do you know if this is true of the high spec gaming towers? (Talking about something costing £1k+)
If it's an off the shelf mass produced high spec one then I'd guess at yes.
If it's one where you spec it yourself and they build it for you then it'll be cheaper doing it yourself.
The problem when you want high spec is that there's always going to be something not quite right so you have to go bespoke. That's when it gets expensive to pay someone else to build it for you.
I don't know why they call it 'building' by the way. You just buy the bits, plug a few wires in and press "on". There's no building involved, just assembling.
Everything's standardised so there's a place for every bit you buy. It's easy but obviously takes a few hours.
The hardest thing by far was deciding what bits to buy. That took ages!! Putting them together was easy.
I really don't think it's possible to go wrong. The wires look complicated but they really aren't. Because they're standardised the motherboard one will only fit the motherboard, the hard drive one will only fit the hard drive and so on....
If you're going high spec I'd do it yourself purely so you can say you have. Even when you realise how simple it was you'll still be proud of yourself and you'll learn a bit as well.