In Australia it's quite common that pre-paid sim cards go on special so it's possible to pick up $30 of credit for only $10, as you can see here: http://www.ozbargain.com.au/node/104698. Taking advantage of these deals to buy apps from Google Play is GREAT because it means you can get $30 worth of apps for only $10 and the devs are still getting paid the full amount (minus the commission to Google of course).
Here's how to do it…
1. On your device, go to Google Play and check out all the paid apps you want to buy and add them to your wishlist. I recommend adding apps (or movies or books etc) to your wishlist any time you see something good so when there's a good deal like this one, you can buy them all at once.
2. Buy the discounted sim card on special (e.g. http://www.ozbargain.com.au/node/104698)
3. Activate the sim card. (For Telstra sim cards, I recommend using Telstra online chat to get it activated so you can be clear about the fact the credit needs to be available for you to purchase app, rather than it being 'allocated' as data which would mean you can't buy the apps.)
4. Insert the sim card into your device, go to 'My wishlist' and select the app(s) you want to buy and when you select 'Buy' you will have the option to pay for the purchase using Telstra pre-paid credit. Select that option. Repeat until you've used all the credit on the sim card (or you can save it to use later, but be aware these prepaid sim cards often expire after a short period of time like 30 days (check when activating it) so best to use it all up straight away so it doesn't get wasted.
5. Take that sim card out and re–insert your usual sim card.
6. Voila! You have all the paid apps you just purchased and it only cost $10 to buy $30 worth of apps.
Enjoy! And please +1 and Share so everyone else can take advantage of this. Remember, the more people buying apps, the better the apps will get.