I would park in one of the NCPs in town, will probably cost you around a fiver for a few hours at Liverpool One, its open late and then just get a taxi in. I go every week to watch Everton play (just got back tonight in fact) and it only costs me about £7-8 which is a similar distance from town. As YSMN said, they do the soccer bus too which is pretty cheap that runs from Sandhills to the grounds. Getting back isn't usually much of a problem, there are loads of taxis about, just walk a bit further away from the ground to avoid the crowds!