Spend £4.99 and get San Andreas & £20 of Amazon coins. Seems a good deal.
If you already have coins you can pay for the game with those and get 2000 back. Free profit.
There are games that give you free coins. You would need 499 for the purchase.
£2 voucher and some of the games mentioned below in the comments no longer work.
Either use coins OR card for payment (ie can't use 200 coins and pay the rest.
Also this game is compatible with many devices, but amazon block installing it on anything that isn't a kindle.
Don't get the apps through the Amazon appstore app because you won't get any coins like I did :(