The thing is, though, you haven't actually flown on a ticket that departs IAD this week and that returns to AMS on April 28th, so checking what that hypothetical ticket would now cost if starting from scratch is not a valid indicator of what price you must now pay to return on April 28 on a different ticket that has already been issued subject to a (potentially very) different set of fare rules.

For one thing, that hypothetical ticket may have access to cheaper fare buckets which are now ruled out for the ticket you committed to last January.

In general - the cheaper the ticket cost initially, the more expensive it's going to be to make any changes. More expensive tickets generally come out of higher fare buckets which will retain better availability (insulating you from fare rises) and generally have greater flexibility (lower/no change fees).

By all means, buy the absolute cheapest ticket if you know you won't have to change it. However, these tickets can be proportionately far more expensive if you then later want to make a change to them. They won't just end up costing the same as if you had originally booked travel for the original outbound and new inbound date from the start - if they did, there would be no point in selling more expensive tickets with greater flexibility, because just buying a cheap ticket would allow you to make any changes at no added penalty.

Cheaper tickets therefore end up being costly when you try to make changes.