The bulbs themselves are not that expensive. Less than $30 on average per bulb. The premade fixtures are costly as they include the ballst (somewhat expensive item), fans (can be expensive or can be cheap) and the metal enclosure (expensive for what it does). If you DIY
your canopy you can usually get the same results for half the price.
Parking lot lights I will use since you mention them. The bulb itself will produce a speciturm that is very different than the MH
bulbs for our usage. The MH
bulbs sold in the aquaruim trade have very specific color spikes in their specitrum. I cant say as much for the high output lights on parking lot poles, etc.
I dont know what the average wattage is on the parking lot fixtures but with MH
you must match the wattage of the ballast with the wattage of the bulb. I would guess most parking lot lights are higher wattage than 175W or 250W. I also suspect those fixutres would not be designed to say cool like the aquarium fixtures so using one over a tank itself could cause heat issues even if the ballast and bulb where compatable.