You're asking two very different questions here
1) Why would anyone spend £100 on a shirt?
and
2) Is there ever justification for spending £100 on a shirt?

1) Two main reasons:
a) They think people will be impressed if they see that they have a £100 shirt, or
b) They genuinely believe that if you spend that much you will get an item proportionally better in cut and quality than a cheaper one.

I suspect that, whether they admit it or not, the majority of £100 shirt buyers fit into the a) category - however there are wealthy City types who buy shirts costing that amount that have no vulgar logo and there is no obvious 'look at me' aspect to their shirts - these are b) people.

2) My opinion is that in some cases there may be justification for a price tag of that magnitude: handmade shirts tailor-made to your precise specifications are pretty labour intensive and bespoke tailoring is really lovely to wear: something that fits you exactly is both rare and quite marvellous.

However paying £100 for an off the peg shirt because of the label makes you a fucking cretin, in my humble opinion.

In my smarter youth I paid high prices for designer clothing and the quality was certainly little better than some non-name brand product. There is a limit to this, however, which my dear brother with his 7-pairs-of-Primark-socks-for-about-a-pound does not see.
( Monty Boyce, My cheese game is strong, Tue 14 Apr 2009, 16:53, Reply)