Eighty bucks... I'd start with adding up what you need and shopping from there.

IMO, three of the newer Crees or SSC's running at 500ma would be the minimum if you're planning on riding at higher speeds (not quite flat out, though) or in the rain at a reduced pace. They're a lot more efficient at lower amperages as well, pushing out well over half of the light you'd get at 1000ma for less than half the wattage. (500ma at 3.3 volts vs 1000ma at 3.6 volts) It's getting cheaper to underdrive and add an led so you can downgrade the battery you'll be needing a notch. Two led's focused tight for distance, one at 10 or 12 degrees for close up.

That would be around 5 watts, plus some losses in wiring and the controller, so I'd round it up to 6 watts just to be conservative.

6 watts by 8 hours = 48 watt hours. If you're using a lead battery, it's also a good conservative thing to double your estimated watt hour requirement to account for capacity loss due to cold and age, while still leaving a little buffer to get you home if you're out longer than anticipated. 96 watt hours ÷ 12 = 8 amp hours. Here's one that's close, at 7.5 amp hours, for $20. Batteryspace chargers run between $13-23. We're up to around $40, give or take, for lead.

Alternatively, you could take a step up to NIMH, all-battery has a pair of 7.2 volt 4.5 ah batteries with a smart charger for $71. That's 64.8 wh; ÷ 48 = 35% surplus capacity, which should be enough with a nickel battery to avoid dangerously deep discharges with some headroom.

So anyway, $40ish for lead battery (5.6 pounds) and charger, $10-12 per led/optic/controller(s), and $10 or so for shipping = Your $80 target budget if you dig up the absolute cheapest stuff you can find and figure out how to house and mount everything from scratch. It's possible, just barely. Just for comparison, you could bump up to a bigger lead battery to run a 10 watt mr16 for 8 hours for maybe five or ten bucks less and four to eight pounds more. led, ftw, imo.

Personally, I'd go for the nickel batteries (0.8 pounds). And I'd step up to a taskled controller for around $30. For about $130 you could have the option of a full power triple led light for the odd fast descent, then turn it down to 350ma through well lit areas or while trudging up hills to compensate.