Until very recently, WT initiates had one of the smallest power ranges in the Westlands, though they didn't know it. Added to that the Tower takes women who can be trained, who are generally less powerful than than those who, like all Damane, have the ability in born.
They sent their best to the WT, and Egwene's use of linking and sa'angreal overwhelmed them. Even if they did take a comparable number of channelers back with them, they've lost both experienced sul'dam and powerful weapons in the damane lost. For that they've traded for a handful of women probably below average strength, many of whom are useless on the battlefield.

Traveling would be a great boon to the Seanchan. It might even have made the cost worthwhile. But don't forget, they were looking for a weapon as well.