Roommate Sim Paying Very Little

So my sim has a roommate and at first he was paying about $100 a week which is pretty reasonable considering our bills are usually around $800. However, for the past few weeks he has been paying $4 instead and I have no idea why. The only major change to the household is that my sim's bf moved in, but I don't think that justifies such a drastic decrease in rent. Is this normal?