This is our third trip, each being 18 months apart. We only have one child. We both work, one car, Sydney mortgage. We live carefully and so we can have holidays. We don't have childcare costs as MIL and FIL drop DD to and pick up from school most days.
I don't think we would be going in Sept this year other than the Qantas SYD DFW specials combined with a good deal at DW. It's a short trip, leave on the Friday, 8 nights. Disney only. About $4k in total including air, accom, tickets, and deluxe dining (we used $1000 worth of points on the flights.) Other than mandatory
factory shopping which I'll get DH work shirts, shoes for DD, etc I don't think we'll spend more than $1500 on the ground, most of which will be shared with SIL - tips, a couple of cab fares, etc. SIL and DN are coming with us this trip, they are paying their own way.
If we had more than one child we wouldn't of had these trips close together.
Australia is expensive to travel around. We went to the Gold Coast at Christmas time. Stayed in an apartment, about a week after the floods. We had specials on the airfares - my MIL and DN came with us as well. Air, accom, car hire, petrol alone was about $2500. We had food, 3 Park Superpass each ($500 total), hire of ECV for MIL and other bits and pieces. Probably spent around $4500k in total. Really expensive for what you get.