Their prices on OF are kinda throwback-y, but the pricing for OmniPlan is probably VERY influenced by how expensive MSP is, or traditionally was.
It's topic drift, but MS Project has done to the critical-path scheduling world what IE did to the browser world. It's worse than the incumbents in lots of material ways, but it's a LOT cheaper and much easier to learn, and so it's sucked up all the oxygen.
I work in enterprise project management / earned value. It USED to be that there were lots of contenders for critical path project management software. Now the last non-MSP product standing is Primavera, and it's somewhat neglected by its new corporate master -- the original owner sold his company to Oracle years ago and Oracle hasn't really done much with it.
So the world at large is, increasingly, stuck with MSP if they want this sort of creature. And MSP, instead of using their position to encourage good practices, or adopt widely-supported features still unaccountably lacking from the product, has instead introduced "features" like Manual Tasks.
What's a Manual Task? It's a task in a schedule that has its dates hardcoded and thus will not move, which is really insanely wrongheaded. A scheduling tool like this is, at a basic level, all about assembling a list of tasks with durations (and, ideally, assigned resources and work and costs) that are connected by a chain of logic. If an early task is delayed 3 weeks, you'll see that in later tasks that depend on the first task.
A "Manual Task" just ignores all that. If this doesn't make immediate sense to you as a horrible idea, let me explain it by way of example. Imagine Excel introduces a feature called Manual Math that allows you to override the rules of arithmetic. If you want the total to say 10,000 but the column has entries that sum to 12,234, just turn on Manual Math!