Good morning Mike. Last season I sold my 1999, 30HP Mariner because I wanted to max out the boat with a 4 stroke. (I bought a 60hp 4stroke tiller) Everyone reckoned that the 30 was worth about $2,500....it had no more than 150 hours on it. It ran great and was quite silent for a 2 stroke, electric start. That motor never let me down it's just that the boat was under powered. My neighbour needed a motor...I went to a dealer and he kindly offered me $1,000 for it. I turned it down and gave it to my neighbour for $1,000 with a set a new spare plugs, a new gas tank (he has no built-in tank) and 3 new props. He's happy as a pig in poop right now and takes it out every other day.
I would say that if that 25hp Yammy is in reasonable condition, it should be worth $2,000. at least.