Hospitals, clinics and medical staff as well as the infrastructure required to support the machines are your limiting factor.
With portable devices that can hook into a laptop or a tablet it's not much of an issue but still there will be a hard limit for the market and it's considerably lower than you would think.
You also need to understand that the lifespan of some of these machines can be decades so in effect if you make it too cheap you either have to build in planned obsolescence or you'll sell yourself out of the market.
It's just not true that multiple competitors have to collude to keep prices well above the perceived minimum costs. Or that collusion is a conscious process, either.