For a good LED, it will be pretty costly. The cheapest I would recommend would be Blackstar with 3 watt diodes. And to replace 400 watts of HID, you would still need about 400 watts of LED.
Maybe check out the GLH 290 watt, they are pretty expensive, like $750, but they are great LEDs, and will perform well.
Other options are a better inline fan to work with your HPS, like the 6" Canmax, that's what I use in my grow tent, and it keeps temps just 5 above outside the tent, and I have a 400 watt HPS, 300 watt LED, 135 watt LED, and 90 watt LED, and a few hundred watts of CFL for side lighting. The fan is like $145, and runs fairly quiet.
You could also get an AC window unit for like $100-$150 with $0.97 shipping from walmart.com
^ Thanks that's exactly the ones I saw... they're weren't cheap either.
I have a 420watt HPS, but Heats and ventilation is a serious issue, so I been playing with the idea of LED or T5. I like the idea of LED personally - Ill continue to look into it. I'm just scared of the high price / to results ratio - mainly I want a low heat light that performs well in flower time.