I implemented the BTC-e API for Google spreadsheet with so called User Defined Functions (UDFs). Via these functions, it is possible to retrieve all kind of trade data into a Google spreadsheet. It is also possible to give trade orders. The implementation is made in Google JavaScript.

The following functions are available:
1) btceGetTrades(tradePair, nrOfTrades)
2) btceGetDepth(tradePair, nrOfOrders)
3) btceGetTicker(tradePair)
4) btceGetFee(tradePair)
5) btceGetInfo(keyPair)
6) btceGetActiveOrders(keyPair, tradePair, reverseSort)
7) btceGetTransHistory(keyPair, count, dateStart, dateEnd, reverseSort)
8) btceGetTradeHistory(keyPair, tradePair, count, dateStart, dateEnd, reverseSort)
9) btceTrade(keyPair, nonce, tradePair, type, rate, amount)
10) btceCancelOrder(keyPair, orderId)

I made a spreadsheet which contain examples of the implemented functions.

Regards
Ronald.

You will always buy the "cheapest" sales first. So in your example, if you give an order to buy 6 BTC @ 510$, you will get 1 BTC @ 500$ and 5 BTC @ 501. So you will pay 3005$ in total for 6 BTC.