On a side note, what price do you think this is best at Tristan?
I have no idea, high cost cards are hard to balance. Wero is probably right to start to playtest this at 8. If it turns out to be to strong the right move is probably to nerf the card, not to make it Platin or Colony level expensive.
Usually you get Prince during the middlegame and play it the first time somehow around 2/3 of the game in.
So Admiral will also be permanently in play fairly late and while gaining and Summon-style autoplaying cards will still be good it will not be as good as early in the game. You also gotta care about pile-rushing, especially if the VP count is close. As Admiral gains one extra card per turn it could be detrimental if you are behind but could also help you to three-pile the game and rush towards victory if you are ahead.
All this makes it hard to evaluate this card. While some folks pretend that this is simple and virtually always better than Prince I don't think so.