Was it a small trade?
Commsec adds the price of brokerage into the avg price of the share, when you look at the portfolio.(Not sure if other platforms do this)
So say you made $500 trade @5.7c + $30 brokerage = ~approx avg 6c
If your trade was considerably larger than that, yeah i would be concerned...
They should send you email confirmations, that detail exact price that you bought for.
