Hi Corinne76,
We ended up spending less money than we thought. In Marrakech we ate in restaurants surrounding the Djemaa el Fna, some of which were comparatively more expensive than those not on the square. But still cheap (maybe £6 each). We also ate at the food stalls in the centre of the Square - very cheap (you can eat for not much more than £2.00) . We drank in a couple of hotel bars (one in the medina called Grand-Hotel Tazi at the intersection of Rue Bab Agnaou and Avenue el Mouahidine), as alcohol isn't widely available, or a female may not feel very comfortable in some of the places where it is available. There are a number of restaurants where we found you may get a set meal with wine, but those that we saw were top-end, and may set you back £20-£25 a head. You can get a fast Moroccan food lunch (perhaps spit-grilled meat/chicken) for a quid or so.
I guess what I'm saying is that you can go as cheap as a couple of quid, or spend a lot more in one of the luxary riad restaurants (and you may expect to have to reserve a table).
Shopping in the souk is cheap, as long as you expect to haggle hard in general. I was rather pleased with the large Moroccan lantern I filled my rucksack with for £3 "because it's Friday evening and I want to go to the pictures with my friends"!
I'm sure you'll have fun, and you can get by very cheaply if you want to do so.