Typical fastener torque values for bicycles
I have 2 torque wrenches. One that goes up to 20Nm and another that goes from 20-60(ish)Nm. The little one is necessary for most of the things on my bike like my headset bolts and hollowtech crank bolts (around 7Nm and 14 NM from memory) and the big one is mainly for the cassette (40Nm) and bottom bracket (can't remember) and (just quietly) undoing stuck things (it's my longest spanner, so if I can't undo it with that I need to go get some pipe). 

Unless you're doing super precise stuff with them a lot you'll probably be okay with cheap ones. I got mine when they were on special at Aldi, and probably use them less than once a month, but it's good to have them when you need them. I balked at the cost originally, until I ruined a set of cranks by over-torque-ing the bolts, which cost me quite a bit more than if I'd just got a torque wrench in the first place).