You must purchase a copy for each platform (not for each actual unit).
Assuming both of your ipod(s) are on the same iTunes account, you'll need to purchase (1) ipod/iphone version which will cover both of your ipod touch(s) Apple lets you have up to 5 computers on a single account for just this purpose. Cost=$29.99
You'll also need to purchase the android version of the app, which will cover both of your droids for installation (that's another $29.99)
As for the laptops, there is no charge for using the slingplayer for a pc or mac actual computer, nor is there a charge for using the sling web portal from a web browser; so there is infact no cost for this portion.
So your total cost would be $60.00...