The cheapest plan for offshore is a bit less than that. The $250 Mobile priority plan isn't necessary, as the standard $150 Mobile plan with $2/GB offshore will suffice (I'm using US prices here). The difference is that the more expensive plan gives you 50GB of data before needing to activate the $2/GB option, while the standard plan does not. Overall, the price is the same if one spends all of their time truly offshore, but if the offshore usage is only a fraction of the use on a passage, while the rest of the time is near shore, then the standard Mobile plan is cheaper.
BTW, Starlink plans are priced differently in different countries, so if one is cruising outside the US, it might pay to switch accounts to the cruising area. For example, we are in Guatemala and the equipment here cost us $200, and the plan is $56/month. Mexico is similar cost, and Panama just a bit more.
Mark