score:6
As a rule of thumb, any board, if not written with chalk or easily editable, is outdated.
I have found this site with prices (in Portuguese): Porto de Manaus Agency
Also there's this trip's tips site describing the travel (in Portuguese): Viaje na viajem
You can compare those prices with a regular airliners prices (in Portuguese): Decolar.com
Note getting back by the river is not (necessarily) the same price because the river flows from Manaus to Santarém so Manaus-Santarém is a 30h trip while Santarém-Manaus is a 42h trip.
The Belém leg is longer, so you should expect prices not to be the same. Google maps Manaus-Santarém-Belém by road
I was not able to find an updated site with prices, but I did find some blogs about the Santarém-Belém leg.
(all in Portuguese)
http://www.verdejava.com.br/lugares/rio-amazonas-de-santarem-a-belem/ http://meusegundolar.blogspot.com.br/2012/04/viagem-santarem-belem-de-barco.html http://guiaviajarmelhor.com.br/rota-dos-rios-viagem-de-barco-belem-x-santarem-x-manaus/
If you seek adventure and to blend with the locals it will be a nice trip. Personaly I cannot do that unless by a regular airline because of the (lack of) comfort and time.
And finally, please remember "Couples no allowed in the same hammock" =)