It appears that you can only sell a tiny number of Luxury boxes, but the attendance is steady. I have literally increased my luxury boxes in increments of 2, and the attendance has never declined. Whenever the two new seats are sold, I build another 2. I don't know how many you can build straight away, My very first expansion was to 5002, with just the two luxury boxes, and I have increased a bit ever since.
The courtside seats are similar, but they are less than 2% of my seats. The bleachers are much more volatile, but if they are consistently selling out, you are probably very much under your potential in the more expensive seats. If I were you, I might boost my ticket price on the bleachers, and build some more expensive seats and see how they do.
I typically consider expanding after every TV game, and sometimes toward the end of the month when I am setting my new tickets. If I'm consistently coming up short up capacity, I'll drop my ticket price a bit in that section. If I'm filling a section or coming close, I'll either expand that section or raise the ticket price.
The ratio of seat cost/ticket cost ranges from 12.5 to 16.7 in my sections. That probably isn't an accident, the ROI has to be about the same across all ranges.