You could get away with a basic wrench set (mediuml adjustable, and a few flat open wrenches of various sizes, metric or not depending on the bike) and Allen keys from a hardware store for under $20, or something small like a pack of cigarette, like this one for about $20
http://www.amazon.com/Toolbox-Travel-18-Function-Bicycle-Tool/dp/B000X5YIPKIf you want to do the whole bike maintenance at home yourself, here is what you would need at minimum, but I don't even have it myself, as I prefer to do the big jobs at the local bike shop and use their tools:
4, 5, and 6mm 3-way hex key.
Shimano ISIS cartridge bottom bracket tool.
Shimano cassette tool to remove a freewheel
Crank extractor with 15mm plug.
Spoke tool
Cable tool.
Chain whip.
1.5, 2, 2.5, 3, 4, 5, 6, 8, and 10mm hex key kit.
32 and 36mm headset wrenches.
13, 14, 15, 16, 17, 18, and 19mm cone wrenches.
Torx® 25-star wrench.
15mm pedal wrench.
8 and 10mm forged wrenches.
Medium adjustable wrench.
Tire levers.
Cable stretcher
bottom bracket spanner.
A pin spanner
Chain wheel pegs tool.