In the rainy season you can get a cargo boat from Palestina to Wampusirpi. It's about 800-1000 lempira one way (2 days). In the dry season you cannot do that, only with private boat what is at least 500 US but if the locals don't know you, they will say 1000 dollars and hard to bargain.
From Wampu it's easy to get a boat to Ahuas (4 hours). From there to P. Lempira is a hard job, you have to wait for some other traveller or pay again 300 US for a private one. In every early morning a truck leaves to Leimus where you can get your exit stamp. It's a very hard truck ride without seats. In Leimus you can get some passenger boat to Waspán where you have to spend the night. It's dangerous so be carefull! from Waspán you can leave to Puerto Cabezas frequently, and from there at 1 pm a bus leaves to Managua. This bus ride is the worsat all around in Central America. At least 24 hours with only one driver. Don't waste the time and think about money! Pay a 100 US airplane to the Capital from Waspán or from P. Cabezas. If you go by bus, you will get the stamp in P. Cabezas, if you fly from Waspán, get the stamp in the capital...