Depends on what you consider his first appearance.
Some (including Overstreet) say it's OAAW #81. In G/VG, I'd be thinking $175-$250 range, depending on which way it leans.
Myself, I consider Rock's true first appearance to be in OAAW #83. In G/VG, $225-$300. It's a better book, a tougher book to find, a harder book to get in grade because of the black cover.
Some might say that's too much - but that's the first appearance of the most successful original DC character of the Silver Age. It's an incredibly important book. I paid $440 for a nice, flat VG+/FN- a few years ago. Never thought twice about it. It's an amazing book, and rarer than hen's teeth in any grade. OAAW #81 is at least twice as common.
Hope that helps!
Shep
I ask because I was at a LCS yesterday that had one (#83) for $110...
Jim