At least that's my understanding based on conversations I've had, I've never traded equities.
To be competitive in US equities HFT, you need an FGPA with 40GbE ports hosted in a server (which needs to power and cool the FPGA, and deal with the less latency-sensitive bits of your system). You'll need some storage as well.
That server needs to be co-located with your target exchange(s) matching engines, and connected via 40GbE. You might additionally want remote market data via mm-wave microwave.
You can probably put together a basic but competitive hardware setup for $70k or so, if you ignore redundancy, and you only need to trade a single market. More realistically, you'll need at least two, plus shared storage, and probably more depending on what markets you intend to trade on.
Then you have monthly costs: colocation for the server(s) ($5k-ish+), port fees for the order entry ($500-ish), port fees for market data ($500-ish), physical connectivity fees ($20k-ish) , cross connect fees for the connectivity ($500-ish), wireless connectivity fees, you might need roof access (more fees), market data fees (per exchange), memberships, and trading costs.
I haven't done this for a while, but it easily adds up to $100k per month or more.
So you need to be making quite a bit to pay off your infra, before you start thinking about profit. And your model will age pretty fast, so you'll want to be working on a few possible replacements concurrently.
It's a tough business.
I chased up some actual details from NASDAQ as an example:
http://www.nasdaqtrader.com/Trader.aspx?id=PriceListTrading2