SEO for Small Businesses

I am a small business owner and I don't understand what agencies are charging for per month. I called a few places and got quotes per month all in the couple hundred to $1k per month range. But they won't tell me what their strategy is. Is this normal? That the backlinks are a secret? Is it possible for myself to do offpage SEO by myself? Brian Dean advocates for the low way to get backlinks which I am doing but it is a slow process so far. Thank you!