the lower the price a PR firm is charging you — the more mass email they will use.
Mass email does not usually produce good results. When a journalist decides to cover something they saw in a mass email, they know their co-workers all got the same pitch. They avoid covering these pitches because they would look like an idiot if they wrote an article about something their co-worker did too. They could even get fired for it.
Journalists can always tell when a pitch is a mass email. Most of them use Gmail for Work (google apps, gsuite). These mass pitches go straight to their promotions tab. If they don’t use Gmail, mass email is still so obvious (not personal, does not mention the most recent articles the journalist wrote). If you know anything about email deliverability and how sensitive spam filters are these days, then you know full well that these mass emails likely land in a spam filter.
With that being said, there is a time and a place for mass email during a PR campaign. Small publications will frequently cover products they see in mass emails. However, it is too risky to use mass email at large publications. Yes sometimes (rarely) they will cover stuff they see in mass emails, but it is best to put in the hours of hard work pitching journalists as individuals one by one.