Hi the street vendors and small shops tend to be fake. In the larger supermarkets and Baccrus that also sells alcohol you can buy 200 L n M for 55le - less than £5.50.
In the old terminal there is a small duty free that you can buy on arrival to the left as you come out of immigration - in the baggage hall.
Prices as of last week ( all in $ but just half to get approx cost ) were
L n M $9 for 200
B n H $20 for 200
Marlboro $20
Spirits Litre bottles
Gordons $20
Smirnoff Blue label $14
Teachers $21
Bells $18
Grouse $12
JD $30
Perfume tends to be dearer - Coco Chanel was nearly $100
Gatwick does a deal for 2 litre bottles of spirits for £15.