What would happen?

Most guilds require collateral for anyone running their own characters, and if you steal stuff they don't return it. You can avoid this by letting them run on your character.

Generally it's about 50-100mil, they will take items or mesos whatever is easier.

Just curious, is that standard policy in your world?

I've bought two helms in Khaini so far, and neither of the awesome groups I bought from had such a policy. (That's a good thing, too, because I don't give access to my account for any reason.)