MapKit JS allows 250,000 instantiations and 25,000 service calls per day on the free tier. Currently you must have a paid developer account to use MapKit JS.
I don't get it why they ask for money from people who want to learn their platform. They should be happy more people want to develop for them and charge for actual API usage instead above the free quota.
I don't want to sell anything in the App Store, I just want to display an Apple Mapkit JS Map on my website, but apparently I need a paid developer account for this too, which makes no sense.
Beyond that use this form to request an increase in your limit: https://developer.apple.com/contact/request/mapkitjs/
As for pricing, nothing has been announced at this time.