I suppose it depends on what size you order and there's a section in one of the books about what to syn everything. Last time I checked, a "regular portion" of chips, was something like 22.5 and a smaller portion was 17.5. Not sure what it says in this year's books tho!
I always stay on the 'safe side' and save up at least 25-30 syns to have for special occasions, and roughly add on 5-10 for any extras I might buy.
Hope this helps!