Why do the "Espionage Priority" funds always creep higher on their own?

That is a two fold effect. One, the slider determines what percentage of spending that will be diverted into espionage against that alien, so a 100 BC overall spending budget is worth less espionage per notch than a 1,000 BC budget. Two, increases in espionage ability opens the spending bottleneck by allowing more BC to be dumped in per notch. So, as economy and espionage grow, you go from needing two or three notches in the beginning stages of the game to spend 1 BC to spending 20, 30, or more for one notch late in the game.