I have never had an outboard serviced. But every time I take my vehicle in for service I ask the price of whatever I am having done, say a minor or major tune-up and an additional complaint possibly. I ask them to call me if anything comes up and give them my cell number. Yes, I have had repairs beyond the price of what I originally asked for, but I knew every step of the way what the problem was, what the price would be and when it would be ready. I have used numerous shops over the years and never had a problem really and no big surprises.
New car dealers charge by the job, using GM's time listed for each procedure for example. But there is a built in buffer and I doubt many mechanics take the listed time to do the job. If GM says the job takes 2 hours you get billed for 2 hours and the mechanic finishes faster - sometimes quite a bit faster. The return on an 8 hour day for a shop like this is pretty good.
I think it's best to ask around and find a shop that has a good reputation. If you use them and find they are good stick with them. A good small shop would be my choice over a large one.
But I think learning to do as much as possible yourself is the best solution.