The question is far to broad to give a definitive answer. Without knowing the problem with it, we can't determine how much it will cost.
If it is nonfunctioning due to just having sat a few years, chances are it won't need but a small handful of simple things to be running well, and reliably. If you are at all mechanically inclined, it would be a no brainer, as the parts are cheap, and with a couple hours work, you could have it running reliably.
If it needs a full rebuild, may as well start over.
The problem is going to be finding the right guy to do the work. Most big marinas won't touch it, as at 80 an hour, the repair bill often becomes more than the motor is worth, and the customer then decides to just not pick it up, sticking them with a repair bill. Plus, even if they would touch it, if your marina's/boat dealers are anything like what we have here, I wouldn't let them touch it. Your best bet would be to find the right shadetree mechanic (honest, reliable ones do exist, and I would let one of them touch my engine before the marina does). Someone who works on outboards as a hobby, and working out of his garage will probably do just as good, if not better job than the marina, and likely charge a lot less to do it.
Then it comes down to, what is the motor worth? Technically, it is only worth what the guy with the cash is willing to pay for it. So, you could say, I could sell the motor, after it's running, for 250, in running shape, but it will cost 300 to fix (and I'm pulling these numbers out of my butt, they mean nothing), so it must not be worth it. And, if you were looking to resell the motor, you would be exactly right.
But, let's look at it the other way. If you aren't trying to sell it, what is it's usable worth? Take a look at a brand new motor. The very SECOND you buy it, it looses a large portion of it's value, and suddenly, you've paid more for the motor than you can get for it. So, you look at it like this. Well, it'll cost me 300 to fix, and I could only sell it for 250, but lets see what my 300 would buy me otherwise. If that 300 can buy you a better motor, then it isn't worth it, but if you'd have to add some more cash to said 300, then it may very well be worth it.
And, don't forget to think about the fact that the 300 puts NEW parts on the motor, whereas buying a different used one still has old parts. Goes back to the new motor issue. You pay a bit more for a new motor (new parts), as opposed to a running used motor (used parts), but for a reason.
Now, whenever I mention the value in buying an older motor, I rarely intend for someone to buy it, and immediately put it into service. That is how you get stranded on the water, because there is no way of knowing what has been done to the motor in the past. I, instead, mean to buy an old motor, spruce it up (rebuild carb, new impeller, new plugs, new points and condensers, as well as a good going over). Doesn't cost that much, and you have a motor that is just as reliable as a brand new one, for a fraction of the cost. So consider it that way - you have 300 in a motor that is functionally in real good shape, or you can pay 250, and still need to do a bunch of things to it to get it in good shape. I'll just say that I don't EVER look at buying an older motor without planning on doing the above to it first. THAT, is how you get reliable, older motors, for the fraction of the cost of new.
Old motors don't just leave you stranded on the water because they are old. Instead, they leave you stranded because they weren't cared for. Care for them, and replace things that likely weren't cared for, and you won't be left stranded.
