Visa is $130 in Dakar or $100 in Freetown (double entry).
Guinea is very similar to Salone, but everyone speaks French (except for pretty much everyone in the countryside who only speaks Foulah, and transplants from Salone that speak Krio).
Conakry is hell on earth - never go there. And whatever you do, do not take the direct Gambia taxi (it took 5 days to reach Banjul).
You can either go from Kamakwie to Kindia or Kabala to Faranah. For each, there is supposedly daily transport, but you will probably end up hiking/hitchhiking a good distance. From Faranah and Kindia, there is frequent transport. From Faranah, it is about 5 hours to Mamou, then 3 hours to Labe from there; from Labe, it is about 13 hours to Tambacounda. From Kindia, there is occasional transport to Telimele and onward to Gaoual and Koundara - plan for a day or two to reach Senegal.
Guinea has awesome hiking, rocks, and waterfalls. Dalaba has a somewhat functional ecotourism office with guidance on various hikes. Doucki has amazing hikes led by a very charismatic English-speaking guide; $25 for hike, lodging and all meals.
Dalaba Tourist Office
From what I've heard, getting across GB is pretty effort/time-intensive. The visa is available in Ziguinchor (Senegal) or Freetown for around $30-50 - it supposedly takes only a few minutes.
You must order the visa online (50 euros + service fee). You can pick it up at some of the borders or any embassy. The embassy in Nouakchott told me to come back and retrieve my visa 2 hours later.
From Koundara (Guinea), you will probably have to hitchhike to Medina Gonasse, from where you can get to Tambacounda, Ziguinchor, or Basse in the Gambia. From any of those points, there are tons of transport options to everywhere - from Zig, there is a twice weekly ferry ($28, 15 hours) to Dakar. You can also get a direct car ($28, 13 bumpy hours) from Labe (Guinea) to a town just outside Tambacounda.
Visas are available at the border for $25
Guinea is similar in food/price/culture to Senegal and Guinea, but everyone speaks excellent English. This is awesome.
Basse is a good entry point from Senegal. From there, you can get a 5-hour bus to Banjul for $6.
Visas are available at the border for 50 euros (they only accept euros). You can apparently get them for cheaper in Dakar/Freetown.
Nouakchott is an extremely dusty, exhaust-clogged, mosquito-ridden place. The people are friendly and there is tasty food to be had, but otherwise, it is a rather unpleasant place to be. It is not a good place to walk - you should take taxis everywhere (or your snot will turn black).
It takes about 4 hours (and $10) to get from the border post at Russo to Nouakchott, and about 4 hours (and $15) to get from Nouakchott to the Moroccan border. There is only empty desert and a few pitiful-looking settlements in between.
You can get a taxi to the Senegalese embassy easy enough, but you must order the visa online before going and bring the printout. The Guinean visa is either extremely hard to find or non-existent.
There are a few places in Nouakchott where you can sleep in a rooftop tent for $8. Auberge du Sahara is where all the overlanders hang out, and you might be able to hitch a ride (most are heading south though).
The High Atlases are amazing. You can go skiing for a few bucks a day, or hike for many days through the mountains, staying in incredibly scenic villages. The easiest jumping-off-point is Imlil (about 2 hours from Marrakech by taxi).
The Moroccan cities (Marrakesh, Fez, Meknes, etc.) haven't changed all that much in the last few centuries and are absolutely crazy.
Morocco has great food, cheap accommodation, and easy transport, and is, in general, way easier than West Africa.
Gambia Bird has cheap flights between Freetown and Banjul (Freetown -> Banjul: $165, Banjul -> Freetown: $113) and to Dakar ($170/$190), Accra ($217/$188).
Gambia Bird has Freetown -> London flights for $340 each way.