The parts Amazon is using cost somewhere between $30K and $50K each. Hourly costs will be substantial -- it will likely be cheaper to buy an entry-level development board than to spin up an F1 instance every time you want to run your design.
You can do all of the design and simulation on any EC2 instance with enough memory and cores. You don't have to run the dev toolchain on the target instance type.
Does that mean you can have FPGAs running on multiple F1 instances connected via the PCIe Fabric? It's not clear if this means FPGAs within a single F1 instance, or between multiple F1 instances.
[1]: http://store.digilentinc.com/arty-board-artix-7-fpga-develop...
Personally, I started dabbling with a zynq zedboard. FPGA+Hard arm cores, many stuff on board + extension capabilities.