Not sure your question is the one you would ask to elicit replies that would find out the 'key' motive for buying.
Personally I think you need to use pictures or a collage of pictures or images that relate to 'motives' for buying.
Value - 2 for 1, BOGOF, bigger value packs 100% extra, bargain, first come first served. When you spend OVER £10, receive a voucher for xxxx or 10% off next weeks grocery bill. A range of words or phrases that relate to value.
Offers - All supermarkets will advertise offers on branded items. e.g Kelloggs cornflakes large value pack, this week for £1. Have a look through the press or in leaflets detailing the offers. Some include vouchers for savings.
Fresh - Fresly baked products - smell instore difficult to resist, Salad/veg Organic, fresh from the farm or market, Name of local farm/picture of farmer that produced the items. When people buy fresh they tend to spend more and do so more often. Rather than one visit to the supermarket per week, they might visit twice. There has been a lot in the media about nutrition and family cooking e.g Jamie Oliver programme.
Quality - Brands or items that are a luxury/a treat or are exclusive to the store e.g a product endorsed by a well known chef.
I would tend to keep it simple. Pictures rather than words. Keep any questions short and so they elicit simple hopefully one word answers . What you don't want a presentation to be is a long speech. You need to make it interesting. Pictures, the odd chart and before/after each you make a bullett point statement. i.e I asked 20 consumer which advertising phrase they were most drawn to in regard to value. Then check supermarket advertising for a list of phrases they use to indicate value.
During the presentation don't use too many words, just bullett points or intesting findings/stats. It is only at the end of a presentation, you can make any impact with words, as most people only remember very few parts of any presentation. Remember that if you are making the presentation after many others, the audience might be getting a bit bored, so don't be frightened to be different.
PS. To answer your question, I would not go to the shops expecting to pay £10. I would have a rough idea of the meals for the relevant days and what other other household products are needed. If I see certain items on offer, then I weigh up the value of these and if they represent genuine value, then yes I will spend more than £10. Some shops have started to offer 'family meals' or 'meals for two'. They basically group many items together offering a discount if you buy them all together. A weekend family breakfast for £6, with packet of bacon, pack of sausages, box of eggs. By offering a discount they are encouraging more to be spent.