Nolan,
personally I dont think Marrakesh is more expensive then some of the developments that are appearing throughout Morocco. But if it is, assuming the build costs throughout the country are much the same, presumably land is more expensive (and scarce) in marrakesh than in say Northern coastal resorts.
I think the key to investing in Morocco (or anywhere else for that matter) is making sure you buy a quality product. I have seen developments in Tangiers for under £40k, compared to say developments in Marrakesh mostly from £100k. My research suggested that the cheaper developments are due to a poorer build quality/finish.
Of course, then there are the ridiculously expensive developments all round morrocco like Al Houara near Tangiers and Domain L'Akhdar in marrakesh where villas are in the £millions. Quality definitley, but also location and brand name adds to the cost.
So, when you talk about expensive/cheap you have to be sure you're comparing like with like.
That said, I think Marrakesh developments should be more expensive - it is a proven market, a wealthier local population, an established tourist area, a capital city, cheap flights, frequent flights . . . .