I cycle on a shoestring because I like to pretend that the money I save on bus fares pay for it.
My costs are quite low - the CroMo steel bike I commute on cost £29 (including a new set of brake blocks from Asda) and I commuted on it as bought from
eBay (plus new brake blocks) for a few months, and no doubt it would still be useable in that way (although it would have needed a new chain by now even if I hadn't altered it quite a lot).
In the end I decided I wanted drop bars so I swapped the bars, which meant swapping the brake levers and gear changers and getting a shorter stem, and then I decided that the cantilvers were a pain to adjust so I swapped them as well, and then I bought a new crankset because it's hilly here and the 'granny' ring on the cranks I had wasn't round, and as I was doing that I replaced the chain, and I swapped the tyres for semi slicks, and because I commute on it I bought a rack, a bag and some mudguards and so on. And the old saddle wasn't quite right for me, so I bought one from Aldi for a tenner which is much more comfortable (for me), and the steering was a bit wobbly so I put a new headset in - in all I've spent another £245 (I've just added it all up, it's more than I thought to be honest - but that includes every last nut and bolt - and over £80 of it was to do with changing to drop bars)
However, for a little over £250 I have a strong, reliable bike made of high quality steel (Tange Infinity) that is comfortable and can be ridden on and off road (albeit it isn't road-bike fast nor is it MTB agile) and can carry a useful amount of stuff in all weathers. A similar spec bike new would be getting on for £1000 - although it would have a better paint finish I admit
I'd definitely recommend learning to do your own maintenance. Bikes really aren't that complicated, and not too much can go wrong. I'd recommend sticking to a maximum of 8 speed at the rear as the chains last longer and I would imagine they are less fussy about exact adjustment. I run 7 speed at the back at the moment (although I will swap to 8 when this wears out as 8 speed cassettes are easier to get hold of) and it gives more than enough range. All I get my LBS to do is wheels - that's the one thing I don't yet have the tools, skills or patience to do. Fortunately the wheels on this bike were straight when I bought it, but even then it's only about £10 per wheel for truing them up I think.
And back to the bus fares - I've had the bike about a year, and even though it's cost more than I thought (to make changes that I wanted to do, but didn't have to do) it's cost me £275. Bus fares are £1.70 per day - a cost per year of £408 - so one might argue I have got fitter and bought a bike to my own specification for -£133. I prefer that way of looking at it