Apologies in advance for the VERY long reply!
I think you are called to assume the best in this situation, of both your pastor's daughter, and your pastor's wife. It is not right to assume the worst before even talking to them (particularly of the pastor's wife, who may know nothing about any of this!). Don't let yourself get worked up and feel "cheated". Just assume that somewhere along the way there was a miscommunication, and set about handling it biblically. Even if you discover along the way that the miscommunication was intentional, guard your heart from responding in a way that is contrary to what would bring honor to the Lord.
Some questions asked by PP -
Who ordered the cake? Did they agree to the $200 beforehand?
Whose money was paying for it (you said the pastor's wife signed the check, but was it a church check, or a personal check)? (if personal, why was the pastor paying for his own cake??)
If the church was paying for it and the daughter only knew about the price of the cake after the party, I would just approach the daughter (or whoever agreed to the price) privately and say in a genuinely kind tone, " I just wanted to mention to you that the check you gave me was for $150 instead of the $200 I quoted you. Was there a problem with the $200?" and see what she says. While at the same time trying to gently uncover the story (without trying to embarrass the daughter or her mother). You might uncover a simple miscommunication, or an awkward situation that you can respond graciously to.
Or you might uncover a situation of blatant deceit, greediness, etc. Then what??
You have to look at your own heart and get rid of your own anger, resentment, and offended feelings before you can approach those involved in this situation, and it also means being fully prepared in your own heart to let it go and let love cover this, even if your pastor's daughter and/or her mother are being greedy. And remember that letting it go and letting love cover this means not holding on to any resentment or hurt feelings towards them, and not letting your tongue speak of the situation to others either (especially anyone who knows them). It is easy to look and point out where another person does not measure up. But it is far more important for us to look into our own hearts. You cannot control another person and their heart (or even know it, really!), but you can know your own heart, and work on it.
We like to think hurt feelings are "ok" - who hasn't ever said, "I'm not angry, I'm just hurt"? But honestly, hurt feelings can very EASILY fester into bitterness, even if we say we have forgiven someone. People will say, "But I can't help my feelings! I can't help being hurt." To which I would respond, that you CAN do things to redirect your energies and to guard your heart and your tongue.
None of us know how often we have been (and will be in the future) on the receiving end of love that covers. But we should all assume we have received that covering love a LOT - because we probably have! And we should work to be generous in extending it to others.
When all is said and done, you might also learn to handle things differently from the begining next time. There is nothing unChristian about making up a contract and agreeing on a price beforehand (or declining business or referring potential customers to other bakers in their price range). I think the informality of the agreement this time has contributed to the problem, and maybe more formality might make things a lot smoother all around next time.
I know you are hurt and frustrated. I hope you know I am speaking all of this in love, and only trying to encourage you (not accuse you!)