The helmet: LG Titan carbon.
Cost: Nothing. Got it from my brother earlier this year when he was getting out of bicycling and I was getting back in.
When to replace: next spring or so. It was made in '06, so it's about at the end of its lifespan.

Hah. You think replacing a bicycle helmet is expensive? Take a look at motorcycle helmets: same replacement timeframe (about 5 years or less) and cost from $150 to $800 or more. I stopped feeling quite so bad about gear costs for bicycling when I got into motorcycles.