Is a person's refusal to perform the Hajj obligation due to high costs (which would consume their entire end-of-service bonus) considered a sin or an act of disobedience?

Hajj is an obligation upon the one who is able. So, whoever possesses money in excess of their basic needs is obligated to perform Hajj and incurs sin by abandoning it. As for the poor, it is not obligatory upon them. Hajj is also not obligatory if the travel fare significantly exceeds the customary fare.