Don't put a dime in it. Use it till it falls apart and throw it away save up and buy a better bike when the right deal comes along.

Local thrift stores and yard sales are you best deals. but it takes a lot of leg work. I have dragged some really nice bikes out of thrift stores. Find one in a good area that takes drop offs and hit them a few times on a Saturday if they are processing as the stuff comes in. Last week I got a 1983 Fuji DelRey that once I clean up it will sell for $180. I paid $25 but it needs a full tune up and tires just because of how long it has been sitting. If it has 50 miles on it I would be surprised.
Also look here.
http://portland.craigslist.org/bik/
Find a full rigid or at least a hard tail if you are mostly on hard pack. Suspension soaks up energy that could be used for forward motion. Find a quality mountain or possibly Hybrid from Trek, Cannondale or other better brands. Put a tire on it with a locked center tread. I like Specialized Cross road on the rear and Hemisphere on the front.

This is a good deal on a better quality bike. In fact if you can talk the seller into leaving the fenders (point out that unless his road bike has 26 inch tires those 60mm fenders are not going to work).and most of the hard rocks have rack braze on's. This is boarder line a smoking deal if you fit it.
http://portland.craigslist.org/mlt/bik/1306196427.html
Another not so bad deal if you are short.
http://portland.craigslist.org/mlt/bik/1306100688.html
Good bike idiot seller.
offer $100 no more unless he fixes the tire then offer $110.
http://portland.craigslist.org/mlt/bik/1306044413.html
Nice bike price is high. Watch and see if it comes down.
http://portland.craigslist.org/mlt/bik/1305950087.html
If this is really a cannondale and it is in any sort of shape its a deal. Sounds like they need crack money. Bring friend and even better if they have a gun. offer $20.
http://portland.craigslist.org/mlt/bik/1305895406.html
Decent bikes but price is out of line. Pay no more then $110. I just sold one of these for $80. Check the shifters carefully. they are prone to jamming. REI has new grip shifts for $17. The Shifters off you manga would work.
http://portland.craigslist.org/wsc/bik/1305832062.html
On the higher end. Good bike fair price they may take an offer.
http://portland.craigslist.org/mlt/bik/1305768200.html
Probably where the crack head got the cannondale.
http://portland.craigslist.org/clk/bik/1305749927.html
Decent price
http://portland.craigslist.org/mlt/bik/1305708463.html
Decent bike with decent mid line parts. Price is not out of line but you might be able to get them down around $110.
http://portland.craigslist.org/wsc/bik/1305694369.html
All off those bikes were $400 or more new. Some much more. M500's were around $600 if it has the silk fork. Just make sure you get the bearing serviced even on a garage queen that has been sitting with less then 50 miles on it. Grease dries out and you will ruin the bearings.

Sheldon (RIP) can help you on working on your bike.
www.sheldonbrown.com/