Roddy,
I suggest that you read
How Medicare Drug Plans Use Pharmacies, Formularies, and Common Coverage Rules (PDF file, 456K) to get an overview of how Medicare plans handle prescription drugs.
Then use the
Medicare Prescription Drug Plan Finder. Fill in your state and county under "Learn More About Plans in Your Area" and it will show you a table of prescription drug plans that will be available to you on Medicare. You can click the name of any plan and then click "View plan formulary" to find out coverage details under that plan for promacta, romiplostim, or any other drugs you might be taking.