Yes Beware of Jetstar who fly into Avalon which is HOURS away from Melbourne. Try either webjet or flightcentre, that way it is easier to see price differences. Webjet do charge an extra $30 to book through their site though. Tiger airways are dirt cheap too if your Mum dosnt mind waiting in a tin shed for her flight One way can range from around $60(with tiger) upward. I usually pay around $120. I wouldnt pay mor than $150 personally just because you can get them cheap, you just have to book with plenty of time to spare.
HTH