try this link: www.poloniex.com/balances
But still I feel they should include this as a separate option on their website where we can see our investments, show the price when we bought them and the current value of them etc.
That is what we all want to see I feel.
You just have to go to the specific coin exchange then scroll down.
You will see your history there whether you buy or sell that coin including it's price. No need to revamp the current interface since it was only you who have some concern like that. You just have to be familiar with the current interface like majority of their users and everything will be fine.