So it seems like he'll probably be worth about 5 bWAR this year as a pitcher. Last year was 6, and the year before was 4. That honestly means he has Gerrit Cole value as a pitcher. Cole signed a 9 year 324 million dollar deal. He's also been worth 5, 3.5 and this year it honestly looks like he could wind up with 7 bWAR as a hitter. But even conservatively at 6, he's pretty much a 4.5-5 WAR batter. Similar guys in terms of 1B/DH types Olson and Freeman signed 8/160 and 6/162. Ohtani and is younger than Freeman and is probably worth more than Olson as a hitter and is just going to get paid more as a hitter. If his arm ever falls apart and you really have to decide to make him a hitter, he'll gain value cause he can play in the outfield again.
It's really hard to quantify what an Ohtani contract should look like cause 500 million does seem to be what he's worth. He'll undoubtedly increase the number of fans watching games, and sales of merch as well. The otherside of it is, because there's never been anyone like him, there's no way to no how long he can do both without deteriorating health wise. He hasn't so far, and for three years has been remarkably solid. But whatever contract it is, is probably going to contain some incentives that kick in for both managing to pitch and hit in the same year maybe? No idea honestly, it's also possible a team just says screw it and gives him the 500 million without any of that, but for say 10 years and 500 million that could feel a bit risky even for Ohtani.