FWIW, I did a test buy and sell trade in Fidelity, and the 1st 10K shares is $53.00, then $50.00 more for every 10K shares added.

Both limit and market orders were the same price, so approx 5% of the order value at 11 cents, and I've bought at 15 cents a while back and the cost was still $53.00...a flat rate.

50K shares sold at $12.00 = $253.00...LOL.

The cost varies based on ones total portfolio value.

Ameritrade is a good deal.