It was recently estimated by VADA that roughly 375 UFC fighters could be random blood and urine tested twice each year, with annual costs of somewhere between $1 to 1.5 million dollars.

Max Boxing - News - Azad Championship Report - Floyd Mayweather and the new wave of drug testing in boxing

Test the top-rated thousand boxers a dozen times a year and it's less than twenty million a year. Pay for it by billing each individual promotion on a sliding scale depending on how much the promotion makes in revenue (obviously Pac Marquez 4 pays a huge slice compared to some non-TV event that features some of the top thousand boxers) and it would work out as a really small promotional expense.