Overall, I've found Southampton very afforadble! I'm from quite a poor background and survived purely off the money from the student loans company and a job here or there while at university for a little extra.
You're probably looking at about £15-£30 per week on food (depending on preferences), £80 a week on halls accommodation, and alcohol and nightlife - well, it's cheap, so how much is up to you!
For accommodation costs, in first year, the best place to look is here
on the accommodation fees pages, ranging from about £75 per week at the cheapest to around £100 per week if you're going for catered. After first year, most student houses go for around £65-£75 per week plus a small amount for bills.
For nightlife, Southampton's got a great selection. There's the union bars and nightclub on campus which are reasonable, then lots of cheap places around the surrounding area. Then you've got the places in Portswood and Bedford Place which are the typical student areas to go, student targetted and therefore CHEAP! Most do ridiculously cheap student nights a couple of nights a week and everywhere has different nights, so you can have it cheap all week - including the legendary Jesters with it's 50p pints and cheapest alcohol you can find
After visiting some of my friends at other universities, I'm always happy to come back to Southampton and it's cheap prices! The night life is cheap, the temptation is to avoid the temptation for curry afterwards

There's some more specifics on the
wiki page if you haven't seen it yet.
For food and living supplies, we've got a massive new Sainsburys that's opened, and Asda, LIDL, ALDI and similar places. There's a great market which comes directly to campus every Monday selling fresh but cheap groceries, vegetables, meats, fish and even cookies, which is incredibly useful! On campus, the students union cafe is generally great for a cheap breakfast, lunch, dinner or snack during the day - just over £2 in the morning for a mega breakfast which fills you for most of the day!
From personal experience, I found it incredibly affordable but all the while not feeling as if I was surviving off nothing - a lot of it is just being wise with money, but I've found Southampton makes this easy

If you'd like any specifics, I'd be happy to go into more detail.
Hope this helps
