for those unfamiliar, the summoning ritual described is that of a gacha game. In a real game you pay money for ingame currencies, usually in the form of a pretty crystal, and you can use them to get a random tool, which could be a character. The pool and the chance of appearance for each characters is known, hence back in chapter 1 they already had a list with names like nobunaga or yoshitsune. For reference, in FGO, the chance of getting the like of nobugana or yoshitsune is 1% meaning they need 1000 of those crystals that probably each costed more than their GDP.