For Unreal from thier website - "A 5% royalty is due only if you are distributing an off-the-shelf product that incorporates Unreal Engine code (such as a game) and the lifetime gross revenue from that product exceeds $1 million USD; in this case, the first $1 million remains royalty-exempt." - much better than Unity's pricing. The > 100k annual only give you Unity Personal whereas you can full powered Unreal at much greater threshold.

https://store.unity.com/compare-plans https://www.unrealengine.com/en-US/license

Oh, I thought you were saying that you couldn't build something in Unity to test the market without paying for it.