Here’s one a user built for under $400. Full spectrum and dimmable, I don’t see the wattage but probably 4-600. It will cover a 4x4 very evenly
If you didn’t want to build your own you could easily snag a couple smaller (2-400W) fixtures from Amazon for well under $700 and cover the same area very nicely. It’s all about the modern LED chips, they’re super efficient and put out a superior light.
You’ll need a new bulb the the hps roughly every year, and a non garbage bulb will be $100. In 5 years when you throw the LED out you’re still ahead a few hundred.