Hell, if you write it in Python then I'll even donate the comments page parser I already have written (including a comment's original marked up plain text recovered from the HTML).
One of many tiny projects that just need a few concentrated days that I've never gotten around to. I suspect you might find quite a few here willing to pay that same $20/year.
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.
The ability to reply is more complicated and probably less useful.