Hi Ken, my name is Jason
The proper course of action here would be;
1. Check to make sure the lower unit has gear oil in it, is full, and has no water in it
2. Remove all of the spark plugs and lightly oil the cylinders.
3. Compression test the engine. From here you would get 1 of 2 results. Either good compresson or bad compression. I will cover good compression under letter A, and what to do with bad compression under letter B
A - Good compression
If the engine has good compression, and the ignition system is showing spark on all cylinders. From here any remaining fuel in the tank would need to be pumped out. New fuel gets added. Change all filters. And attempt to start it up and see what happens. With any engine that sits there is always a chance that the carbs will end up being clogged. And that won't be known until it gets started with fresh gas.
B - Bad compression
There are a couple of things that can cause low compression. One of which is sticky piston rings from the sitting. If compression is low the next step would be to hook up the outboard to a portable fuel tank with fresh gas, run it, and then recheck compression and see if it comes up or not. If it does have sticky rings, in most cases simply running the engine for a little bit will free it up.
Cost wise. If the engine has good compression. After it is up and running the outboard will need some maintainence. The gas has to be pumped out. And it's also a good idea to pump out the oil tank as well and replace it with fresh oil. The outboard will also need new plugs, a gear oil change, and a new water pump impeller installed into it. Figure on 1 full days labor (8 hours) for 1 technician to go through the entire engine and getting it back up into runing condition. And figure on about 200 dollars in parts. And figure on about 4 bucks a gallon for each gallon of old bad gas that needs disposal.