Instead of giving a discount from 10 to 5 after selling 60 000 pc, you can apply a surcharge to add to the base price of 5 another 5 USD, which will be applied for the first 60 000 PC.
And then you can just use the standard condition update functionality with set in the condition record Max.cond.base value = 60 000 PC. 
If you really want that this looks like a real discount, then play a bit with statistical conditions.