Per this page, the requirements are $5K min for non day traders and $30K for pattern day traders...
http://www.tradestation.com/fees/stock_margin.shtm
When you calculate your costs, 2 things to consider...
(1) I think you'll really want/need their Radarscreen product and this is $60 more a month
(2) The last time I looked they were only paying about 1% interest vs 4% plus with a firm like Interactive Brokers. I could be wrong about this.
So, if you need to keep $30K in there it is costing you another $75 or so a month so your total costs to use them are really $100+60+75 so you are at about the same price.
You can also do a certain number of round trip trades to eliminate the $150 but you will pay more there then at IB and once again between the lesser interest amount and the greater fees per trade it all comes out about the same. One way or the other my impression is that you end up paying $200 to $250 more than you would if you were using IB.
I've used the product in the past and think its a great platform so if you need the features it may be well worth that fee.
Hope that helps, although I can't promise it's 100% accurate at this point.