When I grab the hacks again I will start up free lobbies; again; so if you stick around untill then, no more than a month, mine will start up. But as of now it seems like no-one is doing it for free, if anyone is give me a buzz.
What happens is that the hacker gets all of the kills and helps complete the level, but the people who aren't hacking don't get any kills, so their exp is on average 10-40exp and 40-100 supply. Also depending on what game mode you are in, such as battle gear and lockdown, where it gives rewards for completing rounds and not just for kills, you can get exp and/or supply that way. In lockdown you only get supply however. Mystery boxes still take into effect where you gain rewards for the amount of boxes collected.
So in short, you get barely any exp and these lobbies are only held for the rewards at the end of the mission. However you can get a large amount of supply by completing lockdown extreme as it gives you medals for every round you have passed.
These rooms are also held because of the trophies you gain when you complete their respectful mission. These trophies only give up-to 2k-10k euros once. Check out which medals/trophies you can get in the 'My Awards' tab after clicking on 'My Info' down in the bottom left-hand corner.