I haven't stayed in hostels so will leave that to someone else. Singapore is going to difficult to do cheaply. Saying that you can eat cheaply at hawker stalls. If you want a night out on the town though Singapore is pretty expensive. Thailand- easy to work on a budget and eating out per day you could do at £10. You could obviously spend a lot more! Vietnam- we were there a couple of years ago and found it really cheap to eat out (Saigon) but it seems that it is perhaps not as cheap nowadays. I would hope you could still get by on £10. Laos and Cambodia- can't help at all but we do have a few "experts" who can help re food and accomodation.
Sounds like a great trip

Hi Chocky
Welcome to HT.
It's a few years since I visited Vietnam and Cambodia but I do remember prices being very cheap. I was in Laos last November, I stayed in Hotels and Guest Houses as part of a pre-paid tour so I can't really help you with prices for accomodation but I believe you can stay in hostels for around £7-£10per night. Places like Luang Prabang and Vientiane are a bit more expensive, have a look on http://www.hostelworld.com and http://www.hostelbooker.com for details of some hostels and prices.
I usually ate in local restaurants midday and could get a bowl of noodle soup for around £1 and a beer(litre size) was 50p to £1.00. To eat in more touristy restaurants the prices would cost about double. In the evening dinner would cost around £2-£4 including a beer. I would say that eating at local restaurants you could get by on £5 per day for meals. There were restaurants that were more pricey in the larger towns but you could usually find small local places that were really cheap. Of course the pound has lost more value in the last couple of months so prices will seem more expensive now.
A good site for finding out Laos information is Trip Advisor http://www.tripadvisor.com I found out loads of facts about travelling in the country from them. Lonely Planet's Thorntree usually has loads of up to date Laos information.
You might like to read my trip reports on Laos in the Far East Forum maybe one of the mods can post a link. .

I always eat Thai food at the small Thai stalls, very fresh cooked food, very cheap and nutritious.
I have even tried crispy frog.