Based on the C-reg operation the company pay for your travel out plus an allowance for friends and family to visit (around £4k worth of flight tickets I total). You get your own 2 bed apartment and a car for the duration of the detachment. I calculated approx £13k additional net if I go compared to if I stay at home for the winter. That's based on around £3k back pay for the summer worked on PPY uplifted to 100% salary, £2k per month in tax free allowance and a bonus of around £2k net when you get home. I am in year 2 of PPY and IF I goto Canada based on my earnings so far this year (with a bit of flexible thrown in) I have calculated I'll make approx £80k gross.
Obviously depends how many of us get to go and it's all based on union and company politics, oh and seniority of course. As others have said the basic in year one with the ridiculous reduction for type rating is hard. However it's a nice place to work, nice aircraft, friendly FD and CC and very professional on the whole. Worse places to be and I've worked for a few airlines in the past.
Hope that helps