I'm not actually convinced anything could make me look good.
ViolentPanda used to say that, not any more. Seriously, get something that really fits you (ie not just at the collar or chest) and it'll work wonders.

Still can't justify it? Suppose you find an "okay I suppose" £3 shirt - it just about fits the dress code, but it soon looks like shit (and you know it), washes & wears adequately, and if you're wearing it to look like you're professional & making an effort it's not v impressive IMHO. You wear it once a month (because you hate it) so over a year, you get 12 wears out of it before probably decidng that you might as well chuck it and get another cheap one. That's 25p a wearing.
OTOH you could bite the bullet and waste your beer money on a £30 shirt - not just any work shirt - but the god of office shirts. It fits, it's a colour & cut you like, and a fabric you're comfortable & smart in.
You feel more confident in it, and others pick up on that. For some unknown reason, people start taking you more seriously. They also ask if you've lost a bit of weight or been working out - some might even comment that they didn't realise how tall you are. Because you like how that shirt makes you feel, you wear it more, not just 12 times a year when you absolutely must, but outside of working hours as well. And because you like it, you keep hold of it for longer (3 or 4 years instead of 1 year). So, instead of 12 wearings, it gets at least 36-48 wears. Even if you only wore it once a month for a few years, that'd be less than £1 per wearing, and it'd make you feel more than 75p better than wearing the cheaper badly fitting shirt that you don't like at all.