Answer by William Gets:
On-site SEO is only really an equaliser – you and your competition can end up doing the exact same amount of on-site SEO; 100%.
Once on-site SEO is done, you will need to beat the competition with great content marketing.
The budget will depend on what you need to beat the competition or how clever you are at creating great content. The better the content, the more it will be shared and interacted with.