You can take it into your nearest Apple Store and they will give you a 10% (I Think) Discount on your next iPod. So take 10% off the price of the iPod you want and that is how much you will pay. You might be able to take it into JB Hi-Fi, EB Games or any gaming store and get a better price for that.
Hope this Helps and please don't buy a Nexus,
There is no trade-in program. There is the 10% recycling credit.
Better to sell your iPod Touch, and use the money to buy the 5th generation.
NOTE: These forums are User-to-User, so you are NOT telling Apple that you will be buying a Nexus 7. If you want to tell Apple, then use the feedback page <http://www.apple.com/feedback/ipodtouch.html>
You can always look at outside sites that accept trade-ins. NextWorth offers competitive prices where you could trade-in your 4th gen iPod Touch and use the money you receive towards the 5th gen iPod Touch. Check em out: http://NextWorth.com
And I agree with @advual - don't get the Nexus 7!