Why would a developer lower their prices? Most people are not aware that Apple takes a cut of all sales. Further, the app developers have already set their prices to maximize revenue. Also, in instances like Amazon, they have already set a cross-platform price that I suspect they won’t want to touch.
Bottom line: I wouldn’t expect many discounts here.
Plenty of them already do. Google's services (YouTube Premium and others), for example, are $5/mo more expensive if you purchase them via Apple IAP. Spotify memberships are 30% more expensive on Apple. There are countless other examples. They just weren't allowed to advertise the cheaper option on iOS until now.
The market ultimately determines the discount, not the company. In a competitive market, some of the gains from Apple’s change will accrue to the consumer, and some will accrue to the developer. What % goes to whom depends on demand elasticity.
Even if a particular list price doesn’t change, I’d expect more frequent and deeper sales.
In a less competitive market for a good or service (due to lack of antitrust enforcement) there should still be discounts, in proportion to the residual competitiveness. E.g. the mobile game market is very competitive, so I’d expect more discounts vs. the video entertainment market where there has been a lot of aggregation.
What you're seeing is companies that can - and many actually do this - offer you different pricing between in app purchasing and their website are now offering you a link to their website where you can sign up for the cheaper price. One notable example of this in practice is Spotify, where its cheaper to sign up on the web than via the app.
I've also heard Netflix has suspended all in app subscriptions and is only going to link to their website for sign ups. I'm unsure if that will translate into savings, but I suspect you're going to see more of this behavior as well.
The 30% / 15% tax is very real and companies that don't have to pay that will be better positioned in the long run, so I imagine even if the price is the same, they'll be able to pocket more revenue doing this as well
I suspect this won't affect games much, except for the exceedingly big ones like Fortnite, but I treat that as a whole separate sector at this point.
Bottom line: I wouldn’t expect many discounts here.