I use Wal-Mart's automotive primer, the name on the cans has changed, but it's about a buck a can. I use light gray, dark gray and also a brownish color. I use it on my metal figures and on plastic models. I don't like paying the modeling companies' prices for their small cans, when the automotive primer works just as well (for me).
I used to use Rust-oleum's primer, before switching to whatever I could find at Wal-Mart, for the same reason-it was cheaper than Tamiya, etc, but worked as well, and had multiple applications, ie, not just for hobby work, but for home improvement-type chores. But it's still around something like $3.99 a can or more.
I found both Rust-oleum and Wal-Mart's primers to be fine-grained, and providing very good tooth for the finished coats.
Hope that helps!
Brad