I would recommend getting either, a 65Watt FLUOREX (or two) or a 85 or 100watt FLUOREX lamp. They kick ass and are, in my opinion, THE light to use when budget or temperature are a factor.
You can get the Fluorex lights that are mentioned in the above post at Home Depot by their flood lights for much cheaper than the auctions listed.
I would just get two 150 reflectors from home depot 6.97 a piece and get two 85 watt bulbs from here www.1000bulbs.com/category.php?category=2086 the cheapest so far
Why would you want the minimum? Do you want the minimum yield? The better question is what is recommended to do the job well for 1-2 plants, in which case I agree with Up4 on the 65w and 85w CFLs as solid budget-conscious choices. The 65w can be found at Home Depot with a fixture, bulb, ballast, everything for about $30 I think. You could also go with multiple lower-wattage CFLs (such as 23w) but by the time you buy bulbs, fixtures, extension cords, etc you will end up in the same place. Get the most light you can afford, not the least.