I think you are mistaken here…Wwise has a free ‘limited commercial license’ which allows you to use no more than 200 media assets, they also offer a free unlimited non-commercial usage.
They offer a few commercial license options if you use more than 200 sounds, the first is based on production budget and another is royalty based. It’s worth checking it out on their site: Pricing | Audiokinetic
Oh and FMOD is completely free if your budget is < $100k
The plug-in costs are add-ons for their software and not UE4 integration costs.