Assuming that your frame/fork, headset and BB, and wheels are in decent condition here is an example of what you could accomplish for approx. $50/mo:

July: Alivio shifter brake lever combination, new cables and housings, new basic grips ($60 +/-) http://www.amazon.com/Shimano-ST-M41...livio+shifters

August: Alivio front and rear derailleurs ($50 +/-) http://www.amazon.com/Shimano-Alivio...vio+derailleur

September: Cassette and chain ($40 +/-) Might want to save up two months and replace at the same time as rear derailleur

October: Alivio crankset ($45 +/-) http://www.amazon.com/Shimano-FC-M43...livio+crankset

November: Alivio brakes ($25) http://www.amazon.com/Shimano-Alivio...=alivio+brakes

December: Ask for any bike bits still needed as Christmas gifts ($0)

Assuming you can do the work yourself, total investment of about $225 gives you new drivetrain, brakes and controls appropriate to the type and level of bike (entry level hybrid). I picked the Alivio components as they are dependable, inexpensive, entry level components that will serve you well if properly cared for. You might not need some of the items like the FD and crankset so you could end up spending a lot less. Of course, the alternative is to save the $225 and put it toward a different bike. Whatever the case you need to start taking better care of your ride.

NOTE: Don't anyone give me a hard time about Amazon vs. LBS. I'm all for supporting local LBSs and the links are just for pricing and illustration. The OP mentioned EBay so he is apparently not opposed to online purchases.