Why do this problem?
This activity is particularly good in a number of mathematical aspects of learning:
Using mathematical ideas and methods to solve "real life" problems
Using and understanding vocabulary and notation related to money
Organising and using data
Choosing and using appropriate number operations and calculation strategies
Explaining methods and reasoning
Making and investigating general statements
The maths behind this scene can obviously be differentiated accordingly, from basic practice using coins and exchanging, to applications of addition and subtraction.
Younger children would enjoy having shops set up in the classroom and being involved in role play. Using empty packets and cartons, along with plastic money or tokens, would be a good way in for many pupils. Another excellent resource would be flyers or leaflets from local supermarkets advertising their offers. (We would have used these ourselves but would get in trouble for copyright
This investigation will stimulate a great deal of relevant discussion. Try to make the most of it! With only a small amount of exemplification, the children themselves should be able to suggest different scenarios which may affect where they would shop. This may lead into "what if?" type extensions, initially instigated by you but later by them too.
At a higher level, a more general approach can be taken, perhaps along the lines of consumer versus retailer benefits. You can open up the investigation further by splitting the class into groups and setting them off on a "shopping project" with certain aims and/or constraints