When you say instances, I am assuming you mean to hook into and actually play with the back-end as opposed to work against the actual services that ArcGIS Server dishes out?
There are some Esri AMI templates available for you to spawn your own AWS instances up to play around with.
Assuming you have an Amazon account setup, you will need to request access ...
Here are some links:
split data to more tables: the less objects to filter, the faster the rendering
We are currently supporting about 30 editors across the country on an m1.large machine in the Northern California region. It costs about $350/month. Our plan is to move to a m3.medium in the Oregon region. This should bring our costs down to less than $200/month. We run standard workgroup version of ArcGIS Server with SqlServer Express, SDE, GIS Services, ...