A very poor job at SEO deserves nothing, a very good job at SEO deserves I would say 25-50% of the "increased" profit. That does not mean residual it means every month if you are doing a good job the profit should be increasing, if the profit increases you get a chunk that is one way to look at it.

Another is hourly, I think in lower scale areas where wages arent high a "decent" inhouse SEO should get $20-$30 per hour, in very high end neighborhoods $30-$60 but really not much more. Inhouse SEO's although maybe gunghoe for knowledge and possessing ability, really cant ask for contract SEO company wages because they lack the resources. One of the reasons I can charge what I charge is because I have so many high power authority sites at my disposal, inhouse SEO's typcially do not. Also inhouse SEO's dont always start out that way, did you? If they are paying you to learn SEO, its hard to turn around and once you've learned it say "ok you paid for me to become an SEO, now I want SEO money". Thats just not the way things work.

I am just thinking outloud here, so please take everything with a 1 am grain of salt. but I say, test the waters in your spare time and start at say $40 per hour locally until you get some sites under your belt, once you have a portfolio and have shown you can do it, and get enough business either quit your current work or ask for more $. SEO is lucrative as a profession if you're good, if you're not paying the rent can be a challenge. And 400 uniques isnt really a gauge, you might have a 100,000 page website that gets that just from being online. What are your target kw's and where do you rank? Have you had positive movement? Etc Etc

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Trying to rank #1 for SEO in Google I'm on the first page but not there yet, if you can link to any page on SEO Guy site with kw SEO (MAinpage preferred) it would be greatly appreciated.