When is asking for an update on PhD admission status appropriate (given international flight prices)
The general rule for you is that you can ask. It would be good to explain in any request that you need to arrange international travel and that finances are an issue. 

But the general rule for the university is that they may be very constrained in what they can tell you. There may be rules or agreements about deadlines and about giving out information early, even if it is available. So, you may not get much of an answer and it would be very unlikely to get an early decision. 

Graduate open houses are useful, but perhaps not necessary in every case. Make reasonable plans but stay flexible and expect that things can change. Money out of pocket probably isn't your most important consideration.