I've read of people doing a SDC to add segments, but haven't tried it myself.

You could go BUF - IAD - EWR for $137 one way and about 4 hrs of time. You might be able to add extra segments by doing a SDC at IAD. IAD - EWR flights route direct, or connect through BOS, CLE, CLT, RIC, RDU, PIT, PWM, SYR, DTW, CMH, and of course BUF. Sounds like you might have an opportunity to SDC and add a few legs to arrive back home with 4+ segments for a day's effort at your $400 budget.