Sounds like a Rear-Main Seal....which is about as you said 300 to 400 to replace.
I just replaced the one on my 94 Accord EX with 212k Miles....so things like that get old...and unless you have more issues with the car than just that, and dont mind keeping that car, by all means dont buy a new one!
(I have my 94 accord that I refuse to get rid of and still bought an 08 Accord EX not long ago....so it really just boils down to what that car means to YOU in value)
Chris
