First, you need to put on the paper what you need (On-site SEO, off-site SEO, how many KWs. Which KWs, where do you wanna rank, which SE you wanna rank, do you want to spy on competition...).
Once you do that then put on the paper what you need for each of these.
Example, for on-site SEO you say that you need:
1. Product description
2. Image optimization
3. home page SEO (meta, title, optimized content...)
4. Site structure
5. Geo targeting etc etc..
After that you put on the paper what you need for off-page SEO...and so on and so on..
If you don't know what you want, ask your provider what will he do for the budget asked, let him put everything on paper.
If someone trow you a random number ( no matter low or high) this means that they will do the same "SEO" job for you that they are doing for every body else, which is kinda not possible. Every site needs personal approach (imo).
On the other side, if you think that someone serious will bring you new clients at $100/month....you must be a client to avoid.