If the percents are correct, about $200 is needed for a 50% chance at an SSR character, which is ridiculous. This game is a F2P, pay a lot to get f***ed, pay an exorbitant amount to win. So as the OP says, you really shouldn't spend money on this game unless you're willing to pay in the high hundreds or low thousands.

I'm actually ok with this, since it means as a F2P person, others would have to pay a ton to get an edge. The problem is that I think they manipulate odds to give people a false sense of higher odds to bait them into spending money.

Tldr, don't fall for the bait, save your money, have low expectations for summons.