i am not sure what you want to do exactly but i believe coinmarketcap can help you with that.
just go to the market of an altcoin like this:
http://coinmarketcap.com/currencies/litecoin/ and under the charts you can click on the "Market cap" and ""price (btc)" to hide these two lines and have only the green line representing the "price(usd)" value.
this may not be the 100% accurate prices in USD you want since coinmarketcap takes an average price among many exchanges.
if you want something 100% accurate then you either need to get the prices of these altcoins directly in USD for example using BTC-E API to get the LTC/USD market data and then calculating your profit loss with some code or something.
and most altcoins don't even have a USD (generally fiat) markets and the value you see is based on bitcoin historical price and is not accurate at all for them.