i got from djibouti to yemen
 

I got to Yemen via cargo boat (dhow) from Djibouti Town. I found the boat after a couple of days by visiting the immigration office (directly in the port) every day. I arrived in a small town on the red sea coast.

Costs to expect:

in Ethiopia:
300,- Birr for the Djibouti Visa (in Addis Ababa)
470,- Birr for the letter of recommendation from your (austrian) consulate (other consulates might very well have different prices)

in Djibouti:
100,- USD for the ticket for yourself and the motorcycle on the boat
up to 40,- USD baksheesh for the guys who operate the crane, translate for you...
If you are asked for freight papers or alike, use your carnet, and be prepared to argue (no baksheesh in this case).

in Yemen:
No Costs at all (even no baksheesh for the guy who operates the crane...)