Paris - Dakar hitchhicking

This sounds like a really cool project. I always look back on my hitchhiking trips and wish I'd documented each of the drivers somehow. They are invariably fantastic people.
I've done a little -- not a lot -- of hitchhiking in Mauritania and Senegal. I felt very safe hitchhiking around Atar because of the massive police presence. When you leave a town you must pass through a police roadblock, give them a fiche, and they radio ahead to the next roadblock to make sure you arrive safely. While I was waiting for a lift at a roadblock near Terjit, I asked a cop if there was any danger of AQIM in that region. He seemed genuinely surprised. "Is that why tourists have stopped coming to this area?" he asked, "sure there's trouble down at the border with Mali, but never up here."
In Senegal I had a very difficult time hitchhiking, so I gave up after a couple days. But I was hitching between small inland villages. I imagine the coastal road between St Louis and Dakar (I've never been that way) would be easy with all the traffic coming down from Morocco and Mauri.
The Western Sahara was straightforward enough, but I had difficulty. There's plenty of traffic, but very few people stopped for me, not even the Europeans. Maybe you'll have better luck than me.
The problem with hitching in these areas is that the environment can be so harsh, and traffic so sparse, that when someone stops and says "I'll take you 5km for 20 euro", the offer can sound very tempting. It takes a lot of willpower to refuse. That's one of the reasons hitching to Mauri was the hardest thing I've ever done.
It's important to print fiche forms before you enter the Western Sahara. These are papers to give to police at checkpoints. They should include a photocopy of your passport as well as your name, occupation, nationality, passport #, plus for Mauritania include visa #, date of entry/exit, and itinerary. I used about 10 of these forms. They're essential to have because they save you time, but they also prevent your driver having to wait 20 minutes while the police write down all your details.

Please do post photos! I'm especially interested in hearing about the leg betwen Atar - Dakar, because I've never been in that region.
You can look at my website if you want to see my whole experience.

I don't remember my exact budget, but I did spend a lot more than I intended. In fact I ran out of money and had to rely on people's charity at the end, because the ATM machines didn't work for me.
I think my hotel in Nouadhibou was 12 euro/night, but that was after I had my taxi driver haggle the price for me. I don't remember how much it cost to camp at Terjit, but it was very cheap. Otherwise, I just wild camped everywhere else. Police always helped me find a safe place to camp. In Choum they insisted I camp in the police station courtyard.
Food costs about the same as in Morocco: baguette, 10 cents, can of sardines, 70 cents, light restaurant meal, a couple euro.
In some scenarios, paying for transport is the only option. For example, there is no road between Choum and Atar, so you have to buy a place in a 4x4 capable of crossing the desert.
Urban taxis around Nouadhibou were very cheap, but everywhere else they were surprisingly expensive, like 500 ouguiya (1 euro) to cross the town.
I was exaggerating about the 20 euros for 5 km, but you must understand that people will usually expect a white tourist to have money. About 50% of drivers will ask for payment in my experience. As a rule of thumb, I offer whatever I think a bus would have cost for the same distance, usually between 1 and 5 euro.

My cards were rejected by all the ATMs I found in Nouadhibou. Your luck may vary, but obviously don't rely on ATMs. Luckily I brought 200 euro to change at the border with the black-market currency dealers. They offer very good exchange rates, but obviously be careful when dealing with them.
I'm sure you can change Moroccan dirham also, but the rates are probably better with Euro.