We do this on all our IPs. To give you an idea, I checked the books for one IP and found 2007/2008 financial year we spent $780 on the yard. That equates to $15 per week. We don't really add on a hard and fast $15 but it really comes down to the house being much more attractive to a tenant if they don't have to do anything in the yard.
My brother does this work (it is his business), he charges us market rates because we claim it on our tax. This particular yard is 907 square metres, so it is a big yard. My brother charges less than some of the franchise type set ups and is very particular, so whilst we could probably save some money by letting the grass get longer between mows, I do know that he says if we do that, he takes longer to do it, so it doesn't really save any money. Also, being my brother, there is no way he would ever rip us off.
If we wanted someone else to do this, we would be careful to check prices carefully.
The same as we don't think "the house has a dishwasher, that is an extra $15, it has air-con, that is an extra $15" we also don't think "lawn mowing, add $15 a week".
It comes down to what the market is prepared to pay. We try for the higher end of any range we are told the house sits in, and for that higher end of what is available, tenants are getting a dishwasher, air-con and all yard work done.
I believe this often swings the deal. When you add in that we allow pets, it is very attractive to many tenants.