I think AWS billing is quite complicated that they probably don't even know what did you get charged specifically for this machine.

You might have leftover reserve instance that applies, which make the listed price inaccurate. That reservation might even be in a different AWS account in the same organization that you don't have access to. That reservation might not even be there between the time you quote and the time you actually launch it if someone/something did launch before you.

Your organization might also have discounts. I believe some discount may also be very confidential. For example, my reseller policy is that the customer must not be able to see AWS Billing in the organization root account as supposedly the price in that console are the price AWS charged the reseller, while we pay listed price minus any discount we negotiated ourselves.

Finally, I suppose they don't want to have prices shown in multiple places as they will need to update it when prices changes. Doesn't want to risk forgetting one place and getting sued for it. You can see that AWS documentations often do not want to mention the price at all, even if that price is currently free.

Chinese clouds kinda make this simple by making reservation part of the buying machine itself - you have to mark that particular machine as monthly/yearly committed when you start it (or convert it later). The complicated part is recycling instances - if you delete a server before its reservation ends it ends up in a recycle bin that you need to look before making new reservations.