While the NFL has discretion over who gets fines, the value of fines was agreed upon by the NFLPA during CBA. The NFL has the latitude to modify from these values in flagrant and "conduct detrimental" cases, but most fines for first offenses follow this schedule.
The defined schedule outlines which infractions and the minimum fine for each infraction for a first and second offense. Taunting (what Spencer got) has a first offense fine of $11,255 this year. Striking/Kicking/Kneeing (what Dayo got) has a first offense fine of $11,817 this year.
Additionally, first offense fines are limited to 10% of a player's weekly cap hit. Dayo has a salary cap hit of $1,965,577 for a weekly value of 109,198.72. 10% of this is how he ended up with a $10,919 fine.