Think about how much it will be worth to your client to be on the 1st page. According tot he product they are selling you can figure a ballpark for it.
Say for example they sell a $2000 course, out of say 500 people that come to their site 1 buys. (very conservative)
Look at the traffic. Say it's 100,000 exact.
That means that 1st place gets about 50% of it, 2nd, 3rd, 4th and 5th places share the 40% and 6, 7, 8, 9, 10 share the rest. On average that would be between 5,500 and 6,500 visitors (very conservative and assuming you don't get 1st place). That means that on average every month he gets between 11 and 13 clients (say 12), which means $24,000 per month just from being on the first page.
Now remember, thats considering a very BAD conversion rate, and a very rough conservative search. Do the figures before you engage them and then ask how much they convert and how much they think it's worth to them. That's something I ask when I talk to clients. Usually owners or marketing managers understand this line of thinking very quickly.
They'll gladly pay you $25,000 for such a service, you just have to have guarantees in place for them to try you out. I mean wouldn't you pay $25,000, then say $2,500 per month to get $25,000 per month consistently? I would.