If the split does not go right out to the panel edges, then duck tape, or plenty of large stickers will achieve a temporary repair. (Stick em on both sides)
Depending on the size of the panel, replacing one panel will be about 50-80 bucks.
If the sail is badly sun damaged (i.e. very crackly and opaque) you need to work out whether its worth repairing as the other panels may be likely to split.