Cost is a big factor obviously, but when you start picking and choosing some family members to go and some not, some for the evening do and others for the full monty, you are storing up difficulties one way or another for the future. If cousins go on one side of the family but not the other, one side of the family will sympathise that you have four cousins and she has three hundred and twelve, but the other side of the family will not.
My two penn'orth FWIW, it's the quality of the relationship that counts, not the numbers on either side of the church aisle. If you want your parents' brothers and sisters and their children to come, it's not unreasonable for Mrs P to want the same. If Mrs P isn't that fussed, maybe you do draw the line somewhere but I'm guessing that the reason for this thread is that it's creating friction to a greater or lesser extent. If it is, and you dig your heels in, it may be an issue for you for a very long time to come.
If I were you, I'd either get your chequebook out and buy her entire family dinner, or you need to start making economies with what you had otherwise planned. Otherwise, there might come a point when you sorely wish you had done for one reason or another.
Besides, she's marrying you Pige. The poor girl obviously needs some kind of break...