Transmission slipping

I've got a 99 4runner, manual transmission.
Today, it started slipping while accelerating in 4th gear.
I think it might have slipped in 2nd about once or twice a few weeks ago.
Is this a definitive indicator that the transmission needs to be replaced?
Or could a transmission fluid flush possibly cure this?
Also, would anyone have a cost estimate for a new transmission?

Thanks,
-steve

Probably you need a new clutch plate.

Thanks Brian.