"'A good grow light costs $100 or so.'"
"$100?
LAMPS PLUS Price $7.99"
Actually it's more like $200 or $300 for a 400 or 430 watt high pressure sodium setup good for about an eight square foot growing area. A 1000 watt setup good for about 20 square feet would cost more. Marijuana has high light requirements, especially when flowering. Marijuana growers seem to have pretty much a consensus that about fifty watts of high pressure sodium light per square foot of flowering space provides for optimum growth. That little incandescent bulb you linked us to, Mojave, would not grow pot. Those are best suited for lighting plastic plants. They really shouldn't even sell those because they don't work. That 65 watt light puts out 610 lumens, less than 9.5 lumens per watt. A 400 watt high pressure sodium lamp puts out around 50,000 lumens, about 125 lumens per watt, and a 1000 watt hps puts out around 145,000 lumens, about 145 lumens per watt.
Aside from just having that light for flowering (growing the buds), they're probably going to need to section off their closet for another light or lights for plants still maturing and one for mother plants they take their cuttings from if they want to grow year around and be assured that they'll have female plants that produce the buds they smoke. They have to have timers and temperature gauges and probably humidity gauges and light proof barriers so they can keep their flowering plants in a 12 hour on and 12 hour off lighting regimen to induce flowering while they grow their mother plants and maturing clones under with longer daily light periods to prevent flowering. Then of course they need proper ventilation and fans to provide some wind to strengthen the plants and keep them relatively dry so buds don't mold. And they need all their nutrients and other junk, and if they are using hydroponics they need aerators and pumps and piping and whatever else they need for that junk. And if they really want to get fancy with really lush plants they'll want a CO2 system to keep CO2 levels high to stimulate growth. It can all run into a lot of money.
Here are a couple of grow shops I Googled up for prices:
http://www.grow-light.com/cgi-bin/index.pl
http://bestgrowlights.com/