Inexpensive costume advice for Gatsby themed party (Male)

I wouldn't get a suit, especially at a freshers event it's likely to get ruined. Most universities would be sensible enough to not have an expensive freshers theme.

Is this for a formal dinner type event or a more drinking type thing?

Smart grey trousers (£20)
Slim fit white shirt (£15)
Braces (£5)
Hat (£5)

You can find braces and hats pretty easily at charity shops. Some of them are probably even authentic 20s.

Just for day to day life you'll usually need a decent white shirt and some smart trousers. You could switch the shirt out for a Henley if it's a less formal thing and you want something a little different.