Triathlons actually vary quite a lot from one to the next. That's great as it means you can line one up that fits your appetite for competition and challenge (as well as fitness).
Harder than the sum of it's parts, Triathlons are brutal endurance races that test the mind and spirit as much as the body. This is not for someone looking to train to lose 5lbs or for a generic goal to aim for. If you're willing to truly dedicate and push past your pre-conceived boundaries, then this is the challenge you need.
2.4km Swim 29km Bike 12.9km Run. Not your usual distances but then again, you are swimming away from one of the world's most famous island prisons! Not the MOST difficult triathlon but still measuring a good 7/10 on the grind scale. The sand ladder is notably tiring. You'll also get to swim past the glowing Golden Gate bridge in the background, which is an experience in itself.
You should be able to complete at least 80% of a full triathlon length that you intend to race prior to the actual event. The last 20% is all blood and guts.
Yes. Providing you go into it well conditioned then you have nothing to worry about. Of course you will be sore, tired and aching perhaps like never before, but it's ultimately safe if you're prepared.
Prices depend, but often people do them for charitable causes so you can race for a cause. Check with your chosen triathlon to budget appropriately.