What is the minimum amount needed to invest in order to join the sale?

For the public sale it is reduced to 10 ETH. During the general sale the minimum will be 0,25 ETH

Those are reasonable amounts. I believe the will attract a lot of investors.

That is a big amount for public sale.Did you mean presale instead of the public sale?

10ETH is for the minimum contribution on Pre-Public round. I suggest that you visit the below link for reference:
https://medium.com/@kubitx/kbx-token-generation-event-a-comprehensive-guide-250ae7e3e656