My fiance and I live in California. We want to ask his pastor, who lives in Kansas City, to marry us. We just found out that he will be bringing his wife along, which we think is great. We planned on paying for his airfare; do we have to pay for hers, too?
No, you are not expected to pay for her airfare. Think of it as a business arrangement -- you are only responsible for the costs of the person who is providing a service. If, however, you can afford it and would like to pay her way, it would be a lovely gesture. It sounds like your pastor has not asked this of you, but it is admirable that you are considering your responsibilities in advance. If he should bring it up, you can simply say that you are on a strict budget and only allotted money for his travel expenses. He will surely understand your financial restrictions.
Wedding Planning Advice