That sounds about right. That should be the Y vs J fare difference, on the low end, and I wouldn't settle for anything less.

Until deferred maintenance starts costing DL real money, nothing will change.

If the aircraft coming in already had the problem, the airport station should have known of this (if they didn't, that's a major operations problem that DL needs to address) and should have called the affected pax and offered a reroute on another airline, or should have proactively moved seats and left the broken seat for NRSA use. That is what other airlines do in such situations. There is no excuse for DL not to do the same.