There are two important variables when placing an order: a price for hashrate and the amount you pay for hashrate.
The price of hashrate is expressed in Bitcoins per hashing speed (MH/s, GH/s, MSol/s,...). An example is 1 BTC for GH/s/day. This doesn't mean that you would pay 1 BTC. You pay 1 BTC only if you mine with 1 GH/s for 24 hours. For example, if you mine with 10 MH/s for 24 hours at the price of 1 BTC for GH/s/day, you pay 0.01 BTC.
The amount you pay for hashrate is determined by you - you won't spend more than you write in that field, but you could spend less since we don't charge you if the pool is not working, if your price is too low or if the order is not active.