I drove from Kinshasa (Bonobo sanctuary) to Brazzaville last week in 2 days. Day 1 I drove until Luozi (had to wait for the ferry for 2 hours, but the villagers were very kind) and slept at the catholic mission in Luozi after registration with the police chief, Freddy. Day 2 we took the allegedly horrible road to Boko, but it wasn't like that at all. It had been dry for a few days and the road was absolutely very doable. Nothing to complain about.
Ferry costs about USD 15 and registration with the police USD 10 (the latter was unavoidable, and may be real to some extent, at least I got a receipt and I was guaranteed not to pay at any other police/immigration office anymore, which proved to be true. Very kind and very easy immigration and custom officials on the way). Very cheap, I would say, if you compare it with the ferry in Kinshasa. If we would have taken that route with our Landcruiser, it would have taken at least USD 600, excluding bribes, potential damage to our LC because of the crane, a lot of trouble with immigration and customs (our visa were issued in Lusaka, which means that they probably would have said that these are invalid), and lots of hassle in general. And probably at least as much time. So take the Luozi route if there is the slightest chance of a dry road.
B.t.w.: we only have had very good experiences in DRC with police/immigration/customs officials and the DRC people. Maybe we were lucky, but it seems to me that to some extent DRC's reputation is unjust.