Plants are lookin' good, definitely increase the light when you decide to flower. Look around and find a cheap 150w HPS, you can find 'em from $50-$100, brand new. You'll be glad you did.
A 150-watt HID light is a very weak light source and while $50.00 is not much to spend, even on something inadequate, if he/she gets to around or about the $100.00 range a $150-watt HID HPS would become a horrible value.
Someone can purchase a new, not the highest tech but still brand new and from a reputable business a 400-watt HID light that will come with an HPS bulb and a MH conversion bulb, higher grade bulbs of each from what is standard are available for an additional price, for $164.95. The reflective hood is not the highest grade dimpled aluminum but it is not a shoddy reflective hood either. It is 15.5" long x 12.5" wide x 6" deep and polished aluminum. The ballast is magnetic instead of digital but for the price and the capabilities it gives it is a heck of a better value than a weak 150-watt HID light that costs around the $100.00 range. The additional expense would be way more than worth it.
If the budget will allow there is a range of different reflective hoods and one of the highest of quality could be ordered with the light system if the person is willing and able to pay the difference.
$119.95 will purchase the same basic lighting system minus the MH conversion bulb but if someone can find one, as in a MH conversion bulb, at a better price than offered by the light system business the same the same light setup as the first can be had for less.
While not as good of a value a system much like the first one mentioned above, but in 250-watt HID and with a slightly larger reflective hood but otherwise the same can be purchased for $167.95. That makes the 400-watt HID a better value by far unless it would cause heat issues that for whatever reason could not be dealt with.
The same basic 250-watt HID light but without the MH conversion bulbs sells for $117.95, again making the same basic 400-watt system a better value but again if a 400-watt would cause issues that could not be overcome the $117.95 could have a MH conversion bulb added, again if able to be found elsewhere at a lower price than when included with the upgraded system, and again you have more wattage for not much difference in price than the upper ranges of what you considered to be an acceptable value for a 150-watt HID light and likely have a superior light.
If someone wanted a very good value, and there were no reasons why they could not purchase one, you can purchase a 400-watt digital ballast that is switchable so an actual MH bulb could be used instead of a conversion bulb, and of course also burn a HPS bulb, they offer one for $229.95, plus of course any upgrades of reflective hoods or higher grade or extra bulbs etc. The basic system is more than sufficient as it is. Changes would only need to be made for reasons of size/shape or reflective hood for the size and shape of the area it would be used in, if additional ventilation were to be needed or if someone just wanted more and or better. The basic system comes with a larger higher grade reflective hood that is one of the optional upgrade hoods for the other systems. In the other light systems it is not an upgrade with a major cost attached, only $35.00, but that brings the price difference between the magnetic ballast/MH conversion bulb model with the upgraded reflective hood to only $65.00. To have a digital ballast that is switchable and will burn a true MH bulb rather than a MH conversion bulb is worth the difference in my opinion.
Of course there might be limiting factors that would effect a decision on what to purchase. It might be total cost and it might be heat issues and it might be any number of different things, but there are far better options and far better values than a 150-watt HID HPS light if it will cost anywhere near $100.00.
If for some reason someone had to stick with a low wattage HID light, like a 150-watt model, a brand new 150-watt light with a magnetic ballast can be purchased brand new for $69.75 and for the same additional $35.00 you can have the upgraded hood that comes standard on the 400-watt digital ballast model. Other higher quality/higher priced/different size & shape reflective hoods can also be picked from depending on what would work best and fit ones budget, just as is the case with all the other light systems I mentioned.
Before you, or anyone purchases anything check out
http://www.htgsupply.com/growlightsubcategories.asp?categoryID=1 You might be surprised how much you can purchase for your money, and have something better ........ and in the end not pay more or at least not pay much more than the price range mentioned for a 150-watt HID HPS light.