This may or may not be that helpful but shows from both sometimes show up on TDF, and NYCB has $30 under 30. FWIW I got tickets to a NY Phil concert once and created a new account to do so and then they sent me a 50% off offer for a future performance (though it was limited to specific shows), but I'm not sure if they do that every time.
Just got an email for 20% off select NY Phil concerts with FALL20
And an email from NYCB with a 30% off code (the five programs mentioned are basically all their fall programs minus Coppelia and the gala):
Celebrate the start of the season with 30% savings on select fall dates in all $184 and $140 sections of the Theater (promotional prices including fees are $132 and $101). Now through September 22, use code 30F25 to take advantage of this Ballet-O-Mania offer on the following five programs.*