What to tip if anything for hotel storing bag?

I plan to stay at budget hotel/pension house/hostel with a room rate of approximately 500 pesos per day for the room.

If they keep my suitcase while I am away; what, if anything, is a reasonable tip for them keeping my bag?

Is it so much per bag, or per day, etc. I don't want to overpay nor do I want to insult with too small a gratuity if required/expected.