Even if it's not 100% reusable it's still likely to be cheaper than New Glenn. New Glenn uses extremely expensive methods of construction. Starship is assembled like a glorified water tank.
I'm not sure I see it that way, I wonder sometimes how much of the expensive construction can be systematized into something less expensive, and how much of Starship's "cheap" construction will remain as they work harder and harder to get to their reuse and turnaround goals. My intuition is that Starship will get more expensive to make over time and New Glenn less expensive but beyond that I can't say if they would meet in the middle or cross.
That said, platform construction costs only dominate when you can't re-use the platform. Anything you can re-use gets amortized over each re-use. That is what had made Falcon 9 so cost effective. Mostly because they get nearly 10 flights per booster.
I guess 10 flights per booster is an average over, say, the whole time since the first booster successfully landed. In the list by this link there are ~70 boosters, and I don't think Falcons flew ~700 missions yet, so 10 flights per booster looks on average approximately correct, even though some - even quite a few - boosters flew significantly more that 10 times.
And recently, many payloads had their booster fly exactly once because the payload needed some extra 'oomph' to get into orbit so the booster was going to fast and to high to re-enter. When you are pricing for an average of 10x booster re-use you need boosters to go much more than 10x because one booster going 1x is going to bring your average down quickly.
I'm looking at overall re-use numbers. Yes, boosters are now being certified for more launches which will, over time, bring both the average and the median number of launches per booster up.
In this conversation, I think the relevant point is that as the 'resusability' of the F9 booster has gone up, so has the cost to make a single booster. That's because they've added things and changed how they make them in order to boost re-usability. I expect the same evolution in Starship/Booster which will increase the unit cost in order to make them more reusable which will lower overall cost of launches because you can amortize those costs across multiple flights.
That said, platform construction costs only dominate when you can't re-use the platform. Anything you can re-use gets amortized over each re-use. That is what had made Falcon 9 so cost effective. Mostly because they get nearly 10 flights per booster.