It depends on who you hire, as well as what your website requires. Factors like your current optimization, website optimization or industry will affect every SEO package, regardless of who you seek help from. The more competitive the market or niche is, the more money you’ll have to pay.
Other than that, some companies will also charge based on their results. A proven track record is the evidence that a consultant knows what they’re doing. SEO isn’t just knowledge, but also experience.
A consultant with decades of experience will obviously charge more than someone who does SEO for a year.