I make some of my own clothes, but because I love the process, and get a better fit... Not because I think it's cheaper. It's an absorbing hobby, and you can offset the costs against the clothes you don't buy instead. You should count on a few false starts (and middles, and ends). Don't cut into your vintage Liberty silk until you've made a few practice pieces out of a charity shop duvet covers (which sometimes turn out to be my favourite clothes!)
Patterns are expensive. You can save a lot if you find a newsagent that sells Burda magazine (monthly, ~£5 each issue for about 20 patterns), but the instructions can be hard for a beginner to follow: you could really do with a decent sewing reference book to help you, and you'll need some big sheets of tissue paper to trace onto.
Fabric is also expensive. If you're in the northwest, check out Abakhan fabrics: they do some very good deals in their stores, but be aware that not everything they sell is a bargain. Otherwise, as hinted above, be open minded about sources of fabric... Some of my most-worn, most admired garments have been made out of an old duvet cover!
Before you invest in a machine, please ask around your friends, family and random strangers... One of them may have a machine gathering dust in their loft. You'll need to get it serviced before you switch it on, but if you hit the jackpot with a 1970s or 80s bernina, elna or Viking, you could end up with a couple of hundred quid's worth of machine for the price of a service.