$1.45 per 10k traded seems really high. A good S&P futures strategy (one of the biggest products in the world) is typically going to make 60-80 cents per contract traded. Since each contract is approximately 100k, thats closer to .06-.08 cents per $10k traded? Even for equities (I don't touch US equities, so I'm not entirely sure how good the best strategies perform), making a full price tick per contract is still < $1earned/$10k traded... and good HFT strategies are more on the order of 5-10% of a tick per contract traded.

In any case, capacity is probably the first constraint you hit. Most firms have reasonably accurate simulations, so most HFT strategies are scaled up to as large as they can possibly trade (ie. quote the largest amount passively or aggress with the largest amount you possibly can) within a few days of being released -- once you can confirm that your live trading is at least mostly matching simulation, you usually try to simulate the maximum possible size it can trade and just start live trading that. Since you're typically scalping a tick at a time, your maximum size is typically some fraction of the zero level bid/offer -- relatively small. Typically the way you scale up is either have better execution (know when to size up/size down appropriately) or better prediction quality -- since you're adversely selected, your bad trades get filled at a much higher percentage than your good trades, so as your have better prediction quality, a smaller percentage of your volume is bad and you can start to fire larger and larger.