It really depends on what work you want to do and if you're referring to regular maintenance or just emergency repairs. I started off with nothing, turning spokes with an adjustable wrench. That was a pain in the butt so I sprung the $5 for a decent spoke wrench. I didn't feel like paying the LBS to change my cassette so I bought a lockring tool ($7) and made a chain whip($0 with scrap metal and an old chain). When changing my cassette, I needed to break the chain, so I bought a cheapy chain breaker ($4).

It really does come down to what you want to do. I wound up making an adjustable cone wrench, but I can see the benefit of purchasing a set. Eventually I see myself getting the spanners for adjusting headsets and down the line a bottom bracket tool. Eventually, I'll probably be able to do everything I need/want to do. That said, as others have mentioned, there are specific tools for specific things on a bike that other tools can do, but these work better. And then there are tools that are very bike specific (cassette lock ring tool comes to mind).