There are few SEO companies that actually put their prices on their websites and there are numerous ways that different companies bill. Essentially every company offers the same core services of keyword research, on-page SEO which typically is made up of Meta Tag writing, Alt and H1 Tags and content optimization and off-page SEO which is also referred to as link building. It's in the details of work where the prices will vary or the work will vary. Some companies may only optimize a certain amount of pages, research only one theme of keywords or may build more links than other companies (which can actually be a bad thing read more about this at Web Advice).
Below are some examples of our competitors who also are decent enough to offer up pricing on their websites, be wary of the companies unwilling to at least give you a base fee on their website.