So I'm still pretty early on in all this. Just ordered my third mailer over the weekend, so I only have results from the prior 2 at this point.
First mailer went to less than 80 owners, I believe; resulted in one deal; and produced at least 2, maybe 3, "leads" or responses of some sort.
Second mailer went to between 150 and 200 owners; resulted in one deal; and produced 4 or 5 leads.
Third mailer was just over 400 letters, and we'll see what it does. I somewhat qualified/scrubbed the list a bit more this time, so I'm hoping for at least as good response rate. All 3 of my mailers so far have been yellow letters, but I'm thinking about switching to either postcards or more professional looking white letters. Also, the first two mailers had a Google Voice number only. I just added a buying lead website on the third mailer that's going out now.
I'd be curious to know what others' experiences have been in terms of response by phone vs. website, as well.