Approx costs of fuel are (in euros) petrol / diesel per litre:
Morocco 0.95 / 0.68
W Sahara 0.56 / 0.38
Mauritania 0.93 / 0.86
Mali 0.78 / 0.78
Distance Calais to Nouakchott is approx 5000km
I now see if you turn east from Nouakchott to Ayoun el Atrouss (800km) and then south, that road (Ayoun to Nioro du Sahel, in Mali, and to Bamako) is now tarmac (see thread border-crossing-mori-mali-33897 on this Hubb, posted an hour ago).
Also you can get Mauri visa, insurance and local carnet at border from W Sahara/Morocco and Mali visa and insurance plus Laisser Passer in Nouakchott.
I hope this helps,