Hard to give an exact figure without knowing the exact restaurant / ingredients, etc. But as a rough idea, I found the following on the SW website:
Basic, Fish & Chip Shops, Meat Pie, average each = 20 Syns
Basic, Mashed potato, ready made, standard, per 100g = 2.5 Syns
Baring in mind 100g is not a great deal. You could be looking at around 25-30 Syns at a minimum. Don't forget to add more Syns if you have gravy!