There are a handful of Ghanaian tour companies which have websites, from which I chose Jolinaiko Eco Tours.
I had a good experience with Jolinako Eco Tours and can recommend them without hesitation.
I’ve also heard good things about the Easy Track tour company from posters on Lonely Planet’s Thorn Tree Ghana branch. Easy Track also does home stays and a host of business services. One thing that Easy Track offers that I haven’t seen any other Ghanaian tour company advertise is the ability to pay with a credit card or by PayPal, a huge convenience.
The cost for the car and driver was very reasonable. Prices change, so check the tour company’s website. (Don’t forget to budget a tip for your driver, as it’s not included in the price of the tour. See Tipping in Ghana.) Also, because I had the company of a Ghanaian driver all day long, I could ask any kind of cultural questions I wanted. It was like having a personal cultural ambassador for the duration of the trip, a huge advantage if you travel alone like I do.