Look at it this way, complete rolling, braking, front end with a speedometer drive and cable, and a good tire? A decent used tire could go for what $20-30, a good straight wheel maybe $50, forks without pitting and seals that don't leak maybe $65 a good complete brake system, maybe $50
I'd bid $100 and go from there, but $200 for a front end that doesn't need anything wouldn't be out of line. it all depends on the asking price, the quality of the item, and how much you're willing to pay for it. After looking the item over I usually ask the seller to honestly talk about how much he needs for the item. That'll usually get you some insight on how low to go without being insulting.
And if it's a local item on ebay, you can pick the item up to save shipping costs....which can afford you a higher bid.... don't forget craigslist, freecycle, and other local listings, often a complete bike that's been neglected can be had for cheap, part out the rest of the bike and get your front end for free, effectively.