I use something similiar, a 12v, 35watt lamp and a 7.5 ah SLA battery. I can get a solid 90 minutes of bright light. More night riding than I typically do. I also run a tiny little tail light and brake light from the same battery.I just wired up a 35Watt utility light and a dry cell battery from eBay, the kind that are used in security lights. It works great but it's only a 2.2Ah battery so I'll only get about 30 minutes ride time before having to recharge it.
ok, I'll contribute the other way...
Besides being a cheapskate, skinflint, tightwad, I GOT no budget.
Harbor Freight has some 9led flashlights that use 3 AAA batteries on sale for $3 or $4 a PAIR. They are surprisingly bright ! but probably not up to the standards you fellows are talking about... just sayin...
rc