Question: In the Prosperity rules it says "if you have 11 coppers in play and 2 Buys, you could buy a Mint, trash all of your played Treasures, and then buy a Grand Market." My question is: What are you going to buy the Grand Market with if you have trashed all your treasures?

Answer: 
Just because you trashed the coppers doesn't mean you haven't earned the coins for that turn (and can spend them). Their value to you is accrued when you play them.

Per the Prosperity rules you must play all treasures you want to use, before you buy cards. The rules for Mint state:

> 
> If you buy multiple cards in a turn, trash your Treasures right when
>  you buy Mint; you still have any leftover coins they produced for spending
>  on something else.
> 
> 
> 

So the order would be:

1. Play 11 coppers
2. Have 11 coins to spend and 2 buys
3. Purchase a Mint (have 6 coins left to spend and 1 buy)
4. Resolve the Mint card and trash all coppers (still have 6 coins and 1 buy to spend)
5. Purchase Grand Market (valid because no coppers are in play).
The other answer here is incorrect. All your coppers are gone when you buy the Mint; you spent $11 to buy the $5 Mint card and the remaining $6 is squandered.

However, you still *can* buy a Grand Market if you have +$ from the actions you've played. For example, if you've played 3 Festivals for +$6, then you can buy a Grand Market after you buy the Mint and trash all the Coppers.