Depends on the bike you are buying the tools for, in a way...
The way i look at it is the tools should be of a good quality, chances of a Â£30 kit being so is slim. You don't want to be using poorly made tools that will slip, round off or in any way damage your pride and joy.
I never bought a full tool kit for the bike, things like spanners, allen keys, torque wrenches, sockets etc i already had from working on the car. In addition i have:
-Shimano Crank Extractor
-Race Face BB Tool
-Homemade headset press (kinda contrary to what i've said above
-15mm Pedal Spanner
-Some funny Shimano Tool for XTR BB's and Middleburn Spider Lockrings
-Cassette Lockring Tool
I've assembled this lot over the years, rather than in one go... CRC are a good source of cheap (but quality) tools.
The one thing i don't have (and really the most important thing to have) is a bike clamp/stand to support the bike while i'm working on it!