Hello and welcome Andy,
About the chains for getting to Sierra Nevada, it depends on the weather...I have some friends who went there the past weekend and did not have to use them. My guess is that you will not need to use snow chains if you come in March.
About the price, you just have to calculate the rental price plus the 3 hours of fuel for getting there...I would definitely hire a car in Malaga
instead using a transfer to Granada.