I just went through a similar process, tried to silicone (mold appears after awhile), use porcelain repair as advised by Selleys helpline, did not work! Got a few quotes to "resurface" ranging from $600 to $800. A mate who is a building supervisor looked at it and said to check with insurance if they pay out and no point resurfacing the crack as it might "crack" again at a later stage.
Had the tiler to quote ($1495 - includes plumber) and both the tiler and insurance assessor could not believe how the the bathtub could have a crack as it was only 7 years old as we only used it for two toddlers weighing less than 30kgs total for their baths.
Insurance paid out, new bathtub - happy client!