for every subject you take (typically 6 but max 7), you have to do what is called an Internal Assessment (IA), or an equivalent but differently named assignment for languages, and the grade from that is a part of your final grade, along with the final exam grade. idk if this is very different from other systems across the world, but here in Finland at the "regular" high school, only the final exam grade matters for the diploma grade so that was very different for me.
There's also what's called CAS (creativity, action, service), where you have to do 50h of extracurricular stuff for each of those categories, so 150h total. you do this over the 2 years of the diploma programme, and then you have to write short reflections after.
Another big thing is the Extended Essay (EE), which is a 4000 word essay you need to write about a topic of your choice (for example, i wrote mine on the portrayal and representation of mental illnesses in two sitcom shows) and that also gets graded for your final diploma
The last one i can think of now is TOK (Theory of Knowledge), which is an extra subject mandatory for all IB students and is... the definition of wacky, really. think philosophy but on drugs, and specifically about what it means to know something and how knowledge can be defined. it has its own graded assignments, which are notoriously difficult.
the only costs i had were the final exam fees (about 500€), but high school in general is free in finland so that's why. some IB schools around the world have super high tuition fees.
EDIT: oh and a big thing is that the IB is only offered in a few languages worldwide, so despite living in finland and speaking finnish natively, taking the IB meant i had to do high school in english (and study english as if it was my native language instead of a foreign one like i had until that point in middle and elementary school). as a result i have a "bilingual diploma" which acknowledges my fluency in english and would enable me to skip a language test if i were to apply to a university in an english-speaking country.