In a pro tournament you can almost be sure that flash won't be allowed - particularly if you're ringside. A flashgun going off close to the action at the wrong time can be a major distraction for the combatants. In these cases make sure that you completely disable the D50's pop-up flash from firing.
Don't assume that you will be directly ringside - plan for being a bit farther out. I'd definitely have an f/2.8 capable telezoom for this one, since you'll be wanting to freeze the action at shutter speeds of around 1/400 sec. This will mean shooting wide open at higher ISO's such as 800 - 1600.
If you don't have one, either borrow one rent one from
here
Shoot in manual mode and have the shutter speed fixed at 1/400 sec. If possible shoot in RAW mode, ensuring that you have several SD cards that can be changed out between rounds - this will allow you latitude to correct for slight under/over exposure that may occur during the thick of the action.
Since you're going to be shooting with a D50, anticipating the action will be the key element here. The shutter lag and burst frame rate of the D50 precludes "machine-gunning" it, so you'll have to
read the fight and be prepared for the blows to land.
If you can get access in advance to the venue then use it to scout out shooting postions, assess the light and plan for where you can shoot from.
As for what to charge - well, that depends on whom is buying! If you can give me more info on that count I can advise accordingly.