Its very true that whatever amount we give to PILs or even our own parents now, its very hard to lessen the amount or withdraw next time should there be one day we are financially tight. No matter how understanding they are, will still feel something if we lessen or stop giving, just like our pay, we dont expect cuts but only increments.
So maybe 1 win-win solution is dont give PILs allowance but take over certain bills instead?
By paying some bills for them, it frees up some money which they can spend on themselves or save thats equal allowance in a indirect way too. Also the gesture to take over bills will show PILs that you are aware of your moving in will add to their livelihood expenses and you care to share the burden, not simply offloading on them. And another good point is since not giving allowance directly, no worries about lessening or stopping next time. And should one day you have your own house etc, you simply continue paying for your own house's bills, marketing etc which is only right of you.