Actually, it would cost 24,000 gp: 3 * 2 * 2,000 * 2 (because the original spell's duration is measured in minutes).
As for your question, I assume you mean, say, a belt which can be worn normally, but which is always invisible? And, therefore, no one would know I was wearing a belt?
Substantially less than the ability to turn the wearer permanently invisible, I'd say.