Morocco- $47 per day over 15 days. Thanks to Morocco not requiring visas, cheap lodging and transport, and easy do-it-yourself tour options, this is one of the cheapest countries we have visited. We stayed in a few terrible hotels, but also a few rather nice ones, but you could easily stay in nice ones the whole time and not add more than $10/night.
Mauritania- $68 per day over 8 days. This reflects the $45 visa, but is lessened by us staying with a family for a few days. Hotels are a bit pricey in Mauritania for the quality, and most of the food is imported, so it can also be pricey if you want Western-standards. However, transport is cheap, with the iron ore train being almost free.
Senegal- $61 per day over 7 days. Senegal is the other country in this part of the world that does not require visas for Americans (or most Europeans), which reduces the cost. Hotels are reasonable except in Dakar, where you should definitely stay in the very nice Catholic Guesthouse or couchsurf if you are on a budget.
The Gambia- $106 per day over 4 days. This is skewed by the $50 visas and only being there for four days, but in The Gambia, you have to take tours (often costly boat tours) to see much. Guides can be hired very cheaply if you can find them, but boats cannot. We do have some good suggestions for bird and wildlife guides if you decide to go.
Mali- $76 per day over 8.5 days. Visas are $25 each. Mali has more tourists, so everything is a bit more expensive, especially tours of Dogon Country, which are costly, but interesting. If you are really on a budget, you can often get a mattress on someone's roof for very little money, which is actually your only chance to be cool enough to sleep if you are there in hot season unless you pay the outrageous additional amounts (normally $20/night) for air conditioning.
Burkina Faso- $66 per day over 7.5 days. Visas were $20 each at the border, but only good for 7 days. Transport is fairly cheap and often nicer than surrounding countries (even sometimes air conditioned on the main east-west road). Without your own car, you do have to hire a guide and car to see some of the interesting spots in the west of the country, which was not crazy expensive, but raised the average.
Ghana- $46 per day over 14 days. Visas were $30 each. Ghana feels more expensive than it is because at a few touristy places, you feel like you are being overcharged. Food on the street is good (if you can find it without fish sauce) and really cheap. Hotel rooms are very reasonable, though you may want to look at a few in Accra to find the right cleanliness/cost balance.
Togo- $49 per day over 4.5 days. Visas were $30 each. This is completely skewed by us staying entirely with Peace Corps volunteers while in the country. The average would have been about $30 per day more without that. Take the Post Bus, which is nice and reliable (and air conditioned) when traveling.
Benin- $65 per day over 5.5 days. Visas were $20 each. Good value since we had to pay $50 each for the car/tour to Pendjari tour and paid for hotels throughout the country. Take InterCity bus when possible--it was cheaper than most and air conditioned.
Nigeria- $80 per day over 11 days. Visas were $130 each. The visas make it expensive, but our couchsurfing in Lagos offsets that since hotel prices there are very high. Transport costs vary a lot from place to place in the country, sometimes with little reason. Street food availability also varies a lot, and restaurants are expensive. Make sure your hotel has a generator (almost all do) since the country is without power most of the time.
Cameroon- $59 per day over 15 days. Visas were $100 each. Hotels aren't that expensive, though the water and electricity in Cameroon is spotty, so you could find yourself without one or both even at decent hotels (good hotels have generators for power, but the backup for water is to give you a couple of buckets). Travel prices also vary a lot here in different parts, so just bargain hard and hope for the best.
Gabon- $78 per day over 7.5 days. Visas were $100 each. Gabon is expensive. Transport is two or three times more expensive than any other country in the region. Hotels are not cheap, though of a high standard (couchsurfing in Libreville greatly helps our expenses). If you venture to the national parks rather than Lope, they start at $300 per night per person. Yet, we have had a decent time here.
That wraps up expenses. As always, ask if you have any questions.