How much should I ask my live-in relatives to pay us?

We live in a big house (6 bedrooms), and we currently have our relatives (a family with 3 young kids) living with us until they find their own place, probably for a few more months. Our mortgage is $1,950 a month and our utilities are usually around $200 a month. What would be a fair amount (to everybody) to ask our relatives to pay us each month?
Added (1). To clarify: no, the relatives are not dead-beats. Yes, they are adults and have decent jobs, decent income, and savings. No, we would not be hurting them financially if we take money from them. No, we are not "close" to them and just took them in to help them to "start over" (after a move from a diffent country).

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