Standard non frozen stir fry's are free yes. However, if you get the 'Oriental Frozen Stir Fry Vegetables' from Asda (like I have tonight for dinner) then its 1/2 a syn per 100g cooked.
This one had seasoning apparently-no oil I dont think! It wasn't too bad for value and I added other spices and oyster sauce etc and some chicken, it was quite yummy by the time I finished with it )