The ideal budget for SEO depends on your business size, type, expectations and income. While not always a general rule, many companies assign up to 10% of their marketing budget to SEO, but then again, it depends on the industry they’re part of and their competition.
If you’re running a local company, you can assign a much higher percentage to SEO because you won’t need to bother about other, more expensive alternatives with a higher reach. At the end of the day, you’ll have to go through all your needs and possibilities with a consultant before making a final decision.