LOL! I'm in the exact same situation as the OP: Was looking for a commuter/trainer/beater bike, looked at some new bikes in the $600-800 range, decided it would be cheaper to get a used bike. (Plus that way I could hopefully satisfy my jones for a narrow-tubed lugged steel frame at the same time.)

Found a sweet Bridgestone 600 on eBay, paid $285. Woo-hoo!

Oh yeah, but the tires and tubes and brake pads were all original, 22 years old. So they get replaced. There's another $135.

Fit is kinda funky, so I ordered new bars and stem. $125

And the saddle is this bizarre-shaped hunk of suede which, in addition to being a bit too narrow, gets REALLY HOT! So I bought a new saddle ($110) and will probably get a new seatpost too ($70).

Of course, I'll need new bar tape once the bars arrive. And I'm thinking the DiaCompe brake levers are a bit large, so perhaps some Cane Creek short-reach levers ($76) will be required. For that matter, the original calipers are a bit iffy, so it might be a good idea to replace those with something reliable ($150).

Oh yeah, and then I'll definitely put Speedplay clipless pedals on this bike ($135)

So let's see, that's $285+$135+$125+... Holy crap, I should've bought the new $600-800 bike!!!