Is it obligatory to fulfill a vow made jokingly to sacrifice a calf, especially given its high price and the financial inability to afford it, and with the wife being pregnant?

Whoever utters a vow, their vow is binding, even if in jest. The words of the questioner are among the forms of vows, so he must fulfill it. If he is unable to fulfill the vow after his wife gives birth, then he must offer expiation for an oath.