It always seems that if you add the word party to anything the the price increases. I don’t see why this should be the case. Why should you pay more to celebrate a birthday, engagement or whatever else is your reason for a party?
It can be expensive enough organising a party without having to pay out more to capture the memory of your event in a way that you will be able to treasure for years to come.
For all of my photography I still use the the same equipment and the same experience, so I don’t see why you should pay more just because it’s your party. This is why for all of my sessions I have tried to keep the costs as close as I can without you having to pay more than you need to.