I know I should do an hourly wage, but the amount of time that it would take me either would either be insurmountable amount of shortness, or longness.
As you get more jobs you'll get much quicker at bidding them and giving realistic quotes.. If you underbid, you cannot get mad at anyone except yourself. If you agree to letting clients add features onto a never-ending list of stuff that needs to be done for $250, know that's your own fault and it's your responsibility to practice what you preach and do as you said you'd do. Just an example.
I would ask myself questions like:
Can I build a system like that?
How long would it take me?
Could I hire people who can do some parts more efficiently than myself?
About how many hours would it take to get this done?
What's the minimum amount of time it could take?
What's the maximum amount of time it may take?
About how much would I have to spend on expenses to make this work well for the customer?
About how much time should I plan on putting into this product?
In order to help answer these questions, you can create a visual representation of the product in a mind-map or drawing. Break the project into pieces and make bids on each piece. Add the hours/bids all together when you are done, and show some of your work to the client- of course, the professional parts- nobody wants to see their head technical person playing with mind-maps, colored pencils, and crayons- but that's something we might do when nobody's watching
Lol.. So yeah, you have pretty normal prices for SEO and that- $150/mo sounds like a price I used to charge. Most people don't go for something like that, so I wouldn't center your product around it. I've convinced many people that SEO is important and can make/break a web-site's outlooks for making a profit in the future. Be open to the client's views, which usually doesn't include SEO. Though make sure you thoroughly explain how it can help them.
Oh, and by the way, give all of your secrets away. If they are good enough to help your clients and colleagues, then your clients and colleagues will come back to you! I'm not saying give away all of your "billion-dollar-ideas" but if you know of a white-hat SEO trick that works, tell people about it. Share the wealth of knowledge.
Chances are you'll get more jobs by doing so. I give people advice all the time.. Though it does take a while to make money that way >.> I don't think it helps to hide information all to yourself- especially something like SEO information which may eventually expire as search engines change- as they periodically do.