As Wes mentioned prices vary depending on what type diff you choose.
A True-Trac is very user friendly and fairly effective but it ain’t cheap.
A cone type spring loaded L/S is a nice choice but once they’re worn out they are not rebuildable.
A spring loaded clutch type L/S is also decent but the clutches wear out with age and use and will need replacing from time to time.
Also, on the spring loaded L/S’s they usually are offered in 400lbs. , 600lbs. and 800lbs. preloads with each being more aggressive as the pressure increases so choose the one that will suit your needs best.