Be careful about putting dollar figures on things like this and estimating demand. What if this guy spends a few hundred hours on this and then no-one buys it? Sure, it's his risk to take, and you carefully qualified your last statement, but overestimating demand is a deadly thing. So let me as - why do you think people will be willing to pay for this service, and what kind of numbers are you talking about? 100? 1000? 10000?
It's just an alternative for someone already offering to work for free – I'm certainly not about to write a business plan around it.
My use case is simply that I like my own comments/SMS/tweets/etc. archived and searchable, as it's an easy way of keeping track of stuff with zero effort. In the case of HN it would also serve to avoid the common situation where I miss a reply until hours after the replyer's lunch break ended.