I don't know. Took a look at the
Marine 3000 kit on their website. Unless they have
medical marijuana and opium hidden in there, I don't see anything that would warrant the $700-$800
price tag. It doesn't even mention if its waterproof, which it doesn't seem to be. Sure they have 30 of everything which might come in handy if you want to help a small village, but nothing special in that kit. Things like small wound staplers, IV administer kits, and syringes are not expensive.
Seems you could put together a kit similar to the Marine 3000 by
buying a nice $50 kit and adding a few things. In particular and as others have mentioned, you could augment the kit with items specific to your personal needs and include some things not covered in any of the kits. And for $20 to $50 you can put it all in a nice waterproof box. $80 will buy a larger waterproof case if you really need a first aid kit that large. But as others have mentioned, most hardly use their kits and space is usually at a premium. Although if crossing oceans, who knows what you might need.
I am a weekend sailor for the most part, so admittedly I don't fill the requirements for extended sailing stated by the OP. But I have needed a first aid kit on several occasions. I bought a Costco type kit years ago and tried using that. Having a leaky
boat, I soaked it with poor placement. I then put a kit together myself. I started with a $20 kit and a $20 waterproof box. I added a flashlight, cough medicine, stomach meds, stronger pain relief, eye wash, more butterfly Band-Aids and oral antibiotics. Included trauma size pads, additional space blankets, portable BP cuff, thermometer, stethoscope, scalpel, forceps and sewing needles. I don't think I spent over $50.
Also look around, most communities have free first aid
training from time to time. Being associated with the Boy Scouts since I was a kid, I picked up a lot of
training along the way. Same for people in the
service or connected to the fire department. If not, take the time to read the first aid book that comes with EVERY first aid kit. Worth doing before you need it.