To have a Bath Fitter or other bath tub liner installed can costs from $800.00 to $1,400.00 and although I've heard people say it's cheaper than a new bathtub I would have to say they're wrong since a new bathtub and shower surround can be bought for $500.00 and can be installed by a professional for what a bath tub liner would cost you to be installed.
If it were me I would just replace the whole bathtub instead of just covering up the old.