If I bought an app on my iphone, will it cost to buy it again on a Kindle Fire HD?

Yes, different companies, diff stores. It will cost $ again.

Do you have an Apple device like the iPhone or iPad? If so, you obviously use iTunes and if you were to buy a paid app, and delete it, you will have to pay again to get it UNLESS you just delete it off your device and NOT off your iTunes account. You can delete any app you want off your device and as long as it hasn't been deleted off your iTunes account, you will still have it ready to put back on to your device. Even if you don't like it, I would recommend never to delete it off your iTunes account as you may want it again in the future. If the app has also been deleted off your actual iTunes account, then its gone and you will have to download it again, and if it is a paid app ( where it requires money to be bought ) you WILL have to pay again. I have iPhone 3G and iPad 2 so I know :)

If you have Android or Blackberry, Im not 100% sure unfortuantely but im pretty sure its the same as iTunes. You can delete it off your device ( say your Samsung Galaxy S2 ) but not off your account )whatever that may be! ) unless your prepared to pay for it again.

yes both have different stores iphone has app store and kindle has google play or amazon i think but try 4shared but not all apps work if its minecraft it will.