I'm looking to optimize the cost of our auto-scaling EC2 groups by having them launch spot instances instead of on-demand instances.

What I really want is to be able to keep some servers in the group as on-demand instances, regardless of what happens to the spot instance pricing market. Then I want any additional servers in the group, above my configured minimum, to be spot instances. I'm generally OK with the delay in adding servers via spot requests.

I can't seem to find any way to do this and I've tried to scour the AWS documentation. It appears that an ASG can either be on-demand or spot, but not a hybrid.

I could possibly manually add an on-demand instance to the Elastic Load Balancer assigned to the auto-scaling group, but then the load of that server would not be factored into the auto-scaling measurements and triggers.

I suppose I could enter a ridiculously high bid price in order to ensure that I always get the servers I need, but then I look at the pricing history and see occasional large spikes.

The AWS documentation is at odds with itself, since in one place it says that if you enter a server minimum, that number is "ensured" to be there. But then when you read about spot instances, there are no assurances. The price differential for spot is compelling, so I'd like to leverage that as much as I can while still maintaining an always-on baseline. Is this possible?