This is the 2834th most frequent Catalan word.
costen
"Costen" means "cost" (third-person plural form) in English.
The verb 'costen' (cost) is used here in the third person plural to express the price of the objects being books.
Els llibres costen 20 euros.
The books cost 20 euros.
Here, 'costen' highlights the metaphorical 'cost' in terms of effort, using it similarly to 'require' or 'demand.'
Les tasques d'aquest trimestre costen molt d'esforç.
The assignments of this term take a lot of effort.
'Costen' in this sentence signifies difficulty or a challenging process to complete something, capturing an abstract sense of cost.