I figured out that if you set a tier price higher than the regular price, it will show the regular price anyway. Seems this is the "normal" funcionality, always show the lower price if it's possible.
But I need to change this funcionality, for example set a regular price of $10 for a product and $15 for a customer group X.
How can I do it? I don't know what to modify, I looked for
getFinalPrice functions, but I don't see clearly where it does this logic of get the lower price.