ACARS message cost will depend entirely upon the contractural agreement between your company and their service provider. It would not be unusual for a rate for x nbr of kilo bytes free and then a fee per byte thereafter. At the end of the day the information they convey can save thousands and this when compared to the cost of the message doesn't even bare consideration.
Send the message whenever you must. Those that worry about the cost of an ACARS message clearly haven't got a handle on the bigger economic picture.