You can't over pay for a coin. The exchange will just sell you at the next high price. Example lets says the sell order as follows:

$100.00
$100.50
$101.00
$102.00

If you enter "BUY" 1 coins at say 10000 because you didn't put the decimal in it will just buy the first coin at 100.00 but if you said buy 3, it will buy the next 3 for sale but lets say you say buy 50 it will buy 50 coins on the next price up to the amount you have to spend. It WILL NOT skip over all the coins for sale and just sell you 1 coin for 10000.00