But isn't the hardware the costly bit? There's no reason two different software platforms on a dive computer should have different prices.
No. The hardware is lost in the noise compared to fixed costs such as rent, salaries, design, warehousing, marketing etc.
This is why low volume consumer electronics are so hard to do.
The reason the two computers would have different prices would be a marketing one. If the punters expect coping with helium has a value then take it away and they expect to pay less.
Another thing probably going unconsidered is the opportunity cost. At small volumes the screens are a major problem. I don't know how they have solved that at Shearwater but guess they may have a stockpile. Sell your stockpile at a reduced price point and now you have to cope with getting another screen. Did it pay off?