The average box set is $40-50. $90 for 25 episodes is just insane.
Have fun being rich.
Most of us are not rich. We are simply devoted and are willing to save to purchase something.
The reason the DVDs are so expensive is that tokusatsu is a tiny, tiny niche market in the U.S. There is no way that Generation Kikaida could obtain the license from Toei (which is very expensive by the way) and recoup the costs by selling the DVD sets for $40 - $50. That may work with some big series that virtually all Americans know about and is guaranteed to sell thousands of copies, but it isn't going to work with an obscure Japanese series from the 1970s. Also, GK does not do bare bone releases. Every set they come out with has interviews, facts, and lots more that make their releases more expensive.
I own every single release from GK save for Kikaida 01. I'm not rich by any means. If you don't want to pay for the DVDs because you think they're too expensive then that's your choice. However, their prices are fair considering the limited quantities they will sell, the extra features they include, and the considerable cost of obtaining the license from Toei.
BTW, if you check their site around the holidays and at other times, GK regularly has their items reduced by 10% ~ 15%. I picked up all of Kikaida for under $100.