Hey,
Some questions and other things for you to consider.
You didn't include much information. Of course it's always preferable to have a complete sea trial where you can test the engine, transmission, sail handling, etc. However there may be significant costs associated with that.
Generally the BUYER is responsible for all survey costs. This includes a haul out for inspection (if the boat is in the water), or, if the boat is on the hard, the cost of putting the boat into the water and then again to haul it.
If you plan on buying the boat, and then sailing it home, then this cost isn't too much. However, if you're not sure you are going to buy the boat, or if you buy it, will leave it where it is for the winter, then the cost of preparing for a sea trial may not be worth it.
Also, how old is the boat, how much to you trust the seller, and what condition is everything in?
If the boat is pretty new (less than 10 years) and has been recently used (like this season), and I was going to leave the boat where it is until next year, then I would not pay for a sea trial. i would have the boat surveyed. and I would insist the engine be run on land. This is pretty easy to do with a bucket and a hose. You can run the engine, let it get to up to temperature, make sure it doesn't overheat but you can't load test it.
if the boat is older, hasn't been used in some time, then I would probably pay for a proper sea trial.
There are always exceptions of course. I bought a boat that was on the hard. The owner's storage agreement included launching. I planned on sailing the boat home, so I had the boat rigged and launched at no cost to me. I paid for the survey and sea trial (to my surveyor). If I rejected the boat I would have had to pay for the de-rig and haul-out.
Decisions decisions.
Barry