Quid est merces?

I’d be grateful if anyone can take a quick look at this question and answer from Orberg LLPSI Exercitia Latina and let me know if it looks OK. I’m not clear sometimes on the case of relative pronouns - I’ve put the pronoun here into acc. - which I’m pretty sure is right. Maybe someone can let me know if I’ve got this right - or anything else wrong..

Quid est merces?

Merces est pecunia quam discipuli Magistro suum Kalendis cuiusque mensis dant.

“Suo” for “suum”, otherwise fine as far as I can see.

Alatius - Doh! That’s great. Many thanks.