Easy. Get drunk before you leave, then you shouldn't even need to buy a drink once you're in the club. Walk to wherever you're going if possible, if not fill the taxi so it should only cost £1 each if you live close to town. Go to student nights as they tend to be cheaper and find the best drinks offers - most places will have regular prices on a lot of drinks but will be selling cheap vodka/pints as well so buy these. See if there is a guest list on Facebook for the night so you can get in free.
I rarely spend £20 on a night out unless it involves getting a taxi for a fair distance. Maybe I'm just lucky I get drunk easily? Pre-drinking is the most important thing though.