Our youngest son left home to go live with his fiancee?

just over a year ago, jess has since lost b her job and had to take a lower pald job, hence they cannot afford to keep their apartment on, we have agreed that they can come stay with us until they have saved up enough money to go to australia to visit jess,s parents and to try get some work, they have offered us 75 pounds a mth each to live here, which im not sure is a reasonable amount, now the time is nearly here for them to move in im dreading it as we have got used to our own space, love my son dearly but just not sure how this will all turn out. as we still have our eldest son living at home too, ant tips most welcome please.