Every cultist you get it increases the price of each of them by ten.
So, if you have zero cultist it is, the price of ten would be:
10+20+30+40+50+60+70+80+90+100 = 550
and if you have 1:
20+30+40+50+60+70+80+90+100+110 = 650
and if you have 10:
110+120+130+140+150+160+170+180+190+200 = 1550
So, the price is the same for both buttons, even if it does not look like it.