Hello,
The prices of ninjas and items vary in different events. Some events have higher prices for ninjas than others, some have lower prices than others. Usually it has to do with the amount and availability of ancillary rewards in those events, though not always. It c*so do with how rare the ninjas are and other factors, such as the value of being able to directly purchase a ninja without dealing with RNG or other annoying things.
Hope this helps, thanks!