Food - I'm not so sure on, being a naturally greedy, fat, git I reckon my eating went down significantly when I was doing a long commute. I also craved healthier foods and reduced on takeaways and suchlike.
Exercise - some people try to offset gym costs against cycling which is only fair if you previously went to a gym and then stopped.
Capital costs - bikes or cars can be done cheaply here but I'd say, if you want to have everything you need, and be comfortable, then an inital £2k budget is about right:-
£1200 - hub gear bike, with disc brakes, dynamo front hub and front and rear lights
£200 - rack, mudguards, panniers, saddlepack, bar bag
£150 - stock of spare tubes, basic toolkit and a set of Winter studded tyres(bike needs to have clearance for these)
£450 - cycling clobber and shoes to cover all the seasons and allow you to wear fresh stuff next to your skin daily, or even per commute if lengthy.
Yes you can go a lot cheaper but I'm thinking of someone starting out, lacking the knowledge to make wise 2nd hand purchases and to know what works with what and how to put it together. Personally I'd add another £600 to that to have a general back up bike that could fill in as a commuter in case of mechanicals.
Running costs - you don't have to eat as much as you think for fuel, a hub gear and dynamo lights will greatly reduce running costs. If you replace chains quite regularly, just using cheapo ones, you'll gets tons of life from chainring and sprocket.
Externals - I've already said about gym membership not being needed but there're lots of others. You get to feel smug, you no longer have to worry about not doing any exercise and you get to feel green. Most importantly for me I felt a lot better all round, my mind seemed sharper and more alert at work and I seemed to have a lot more energy in my free time. Even at Dells 20p a mile level I consider all of that an excellent return on investment.