Why exceptionally expensive ? 1vCPU 1GB 50GB, 1 dedicated ipv4, 1 ipv6 /64, costs $60 a year, or 5$ a month, where for example at Vultr it's 6$ a month for the same thing except half the disk capacity.
But it's a project for supporting the development of OpenBSD's hypervisor technology, not a production platform anyway.
He's also ignoring that the operator of the platform is only interested in OpenBSD, whereas on Vultr OpenBSD's ability to run could disappear at a moments notice.
If they remove the ability to work with a custom ISO they would be putting themselves at a severe competitive disadvantage to literally every other competent VPS provider on the market. Doubt they have that in their strategy, I feel it's important myself to be able to partition how I see fit for example.
But it's a project for supporting the development of OpenBSD's hypervisor technology, not a production platform anyway.