It is certainly odd, but I'm guessing this is more about price discrimination. I think price must be greater than or equal to cost at some level, but certainly not proportional to it.
Price discrimination is an important aspect to consider but I don't think it's directly relevant to the post.
From a business perspective, price discrimination probably has more to do with accounting, support, and long-term sustainability.
A transaction that includes 1000 seats requires 1 account to manage them and will likely have in-house support staff to help solve with technical issues.
A 1000 transactions for 1 seat requires 1000 accounts to manage and 1000 potential points of contact that need to be supported when things go wrong.
In addition, companies that have the resources to buy a lot of seats up front are more likely to be more 'stable'. Those are the customers who will standardize their internal processes to use your software, thereby guaranteeing consistent payment over the long term, and higher likelihood of return sales in the future.
That's probably why a lot of SaaS sites offer free accounts for individual/small-team usage. Early adopters tend to be more technically inclined, will evangelize the product if they like it, will be more sympathetic when minor bugs are encountered, and the 'free' aspect means there's no guarantee of support. They're essentially buying a community of QA testers with free seats.
The 1000 seat customers cover the bills (ie CAPEX+Fixed-Costs). The lesser customers cover the rest (ie OPEX+Variable Costs). The latter keeps things in motion, the former signals it's safe to scale up.
To remain profitable it's important to minimize CAPES+Fixed-Costs as much as physically possible because when feast turns to famine they're the hardest to reduce.
In accounting terms, favoring financial liquidity means favoring flexibility to adjust to changes in the market. This is a key deciding factor behind companies shifting to 'the cloud'. Even if the upfront costs are higher than maintaining bare metal servers, it takes little/no effort to ditch redundant infrastructure when it comes time to reduce costs.
