We pay 90% of the time (about 5 - 6 trips back to the UK per year from where we live in central Europe). We have a flight package which covers half of this too.
My Mum chips in for the odd flight for me and DD, maybe once a year - she also visits us about 4-5 times a year, I'm very lucky I know!
I've paid for my younger brother to come out once, as he's a poor student, and DH has paid for a poor friend to visit too, who otherwise wouldn't have been able to come. I'm talking Easyjet flights for around £120 return.
Generally, we see it as a price you pay for living abroad: we moved away, we pay to come back.
Of course, when we do go back, our outgoings are the flights, hire car & petrol, and all the shopping we do stocking up on UK bargains.
Both sides of our families very generously put us up, pay for all of our food, lavish presents on the dc, give us lifts and lend us their car if it's available so we don't always need to hire a car.
And when people visit us, we try to be just as generous and look after them similarly, and keep their costs down, to ensure more future visits!
..... However, the stress, guilt and disruption of being on a plane every month, having lots of visitors every other month, trying to spread yourself and dc equally, takes another, non-financial toll on me. But that's a whole other thread.