The luxury tax this year is $71,748,000. Our projected payroll with Fab Melo would have been $72,531,029 (according to shamsports). That would make us $783,029 over the tax which would have resulted in a tax bill of $1,174,543.
Now we do not have to pay any tax so it still saves us money. Also we are now eligible to receive money from tax paying teams which would mean this trade would net the Celtics even more cash.