Those plants look good and you should get some nice nugs. 600w HPS is a good starter light and you’ll get a decent yield. It’s a much better option than the cheap LED lights you’d find on amazon for the same price. A 600 watt HPS is better for a 3x3. Like the other guy said two 400s (or a single 1000w) would be better for a tent that size.
Since you said the electric bill is a concern for you, once you get a few decent harvests with the HPS you should look at building a nice LED strip rig for that tent. Like 6 4 foot strips evenly spaced and wall to wall. If you built a light with, say, 6 Samsung FB24b strips (or comparable 4 foot strips that run at ~100 watts each), you’d get a much better yield with the same power consumption. My flowering rig is two boards in the middle and strips around the sides. It covers the entire canopy and I only run it at about 400 watts in a 3x3 tent and it produces some fat nugs. All of my lights and fans put together uses under 800 watts. Even in the middle of the summer with the AC blasting my electric bill is about $150 at the worst. Just something to consider down the road. Building a high quality LED light will cost you about a buck a watt all said and done, but will pay for itself in energy savings and increased yields fairly quick.