For up to $300, I'd go for either a 250w or 400w MH/HPS combo like MoG says, with a vented reflector hood hooked up to an exhaust fan, like this
semi-DIY closet exhaust. Don't bother going MH/HPS without some kind of hood and exhaust fan however, temperatures will be way too high and you'll kill your girls.
Otherwise, if you don't wanna stress about temps and you're willing to sacrifice some efficiency and output, you can use an array of CFLs. Use many of them, scatter them closely around your girls. This may be the cheapest option at first, but considering the time it takes setting it all up, managing the canopy, balancing the light distribution, and the myriad of sockets, cords, and bulbs you'll need to pick up, all for a reduced harvest... it may cost you the same or more in the end as MH/HPS.
You might be able to find a good deal on a T5 rig, which are great, more light then CFLs and less heat then MH/HPS, but likely too expensive for your budget.