It should be allowed to use Unity asset store items that you paid for in all UE4 projects or CryEngine, even commercially. Same the other way around.
Agree that the price difference is somewhat strange, but with more items, prices will also fall in the UE4 marketplace.
Right now, like the first couple of years, is simply gold rush time for sellers to ask for high prices when a new engine is out and where content is rare.
Epic is also happy with that because of the 30% share. You can actually make the same amount of money in this situation with a marketplace item than with an indie game, which is a bit ridiculous.
We recently bought a little submarine model on the asset store for just $5 or so as I remember to use it for testing in our UE4 projects. There was no boat model on the UE4 marketplace. But even then would have bought the Asset Store item instead because it is cheaper. On the UE4 marketplace this would have been at least $30 or something or more. Not so good.