I did not prepay and was lucky a group was crossing at the same time I was. I paid 150.

Everything is negotiable in Africa, I'd imagine prepaying might drop the price further

I will be doing this mid March entering Morocco from Spain around14th March in a 2003 Jeep alone. How do you prepay? Or should I just play it by ear alone or wait for a group to come along and go through with them?

I will be doing this mid March entering Morocco from Spain around14th March in a 2003 Jeep alone. How do you prepay? Or should I just play it by ear alone or wait for a group to come along and go through with them?

crossing into Morocco from Spain your best bet is Tangiers Med,
you don't need to pay anything when you cross from Spain,

its further down crossing from Mauritania into Senegal that one has problems, Avoid the crossing at Rosso as its basically bandit country, the crossing at Diama is much easier however you will have to pay quite a bit for a Passavant, I have had advise from various people that say you can contact someone beforehand and negotiate the "fee"!
if you need the details let me know and I can PM them to you.

The details would be very much appreciated. Thank you for your help

PM sent