How much, generally, would people pay to have an appointment with a potential client?

Me personally, would pay $500/lead.

Depends on quality though.

Max. $2000/lead. Min. $100/lead.

You may want to clarify as to what your defintion of a lead is...otherwise you may be responsible for this kid quitting school thinking he can earn $14K a week by cold calling!

Let's group businesses into four categories based on how many clients they get and whether their goods or services would be considered as a major cost, to the average consumer.

1) Low cost, low volume (e.g.- hairdresser)
2) Low cost, high volume (e.g.- restaurant)
3) High cost, low volume (e.g.- accountant)
4) High cost, high volume (e.g.- construction contractor, plumber, roofer, electrician, chiropracteur, dentist, jeweler)
(retail stores can be grouped into varying categories, depending on what they sell)

You'll notice that I gave the most examples for category 4.
This is because they generate the most SOLID leads- by far.

It's difficult for a hairdresser to justify giving an SEO specialist a thousand dollars: he has to cut a lot of hair to make that money back.
On the flip-side of the coin, a dentist can make this money back after servicing only a a couple of clients.

Category 3's are important as well, an accountant, lawyer, interior designer, etc. is an excellent referral source. They end up sending a lot of business your way.

Now, back to terminology.

Strong lead = A recurring lead. A client that consistently pays for SEO services, month after month.
Weak lead = A client that leaves after the first month, or first few months.

Note that category 2's and 4's are usually strong, 1's and 3's are usually weak.

I happen to be rather good at selling myself, but if I were not, I would pay:

$400 for category 4 strong leads.
$200 for category 4 weak leads.
$500 for category 3 strong leads.
$200 for category 3 weak leads (most 3rd category folks are weak leads, they don't require massive SEO efforts).
$200 for category 2 strong leads.
$50 for everyone else.

You may have realized that it takes a month to determine whether a client is strong or weak. This means that you should get a certain payment first (perhaps a weak client flat rate, dependant on category) and the rest upon the clients' second payement.

But these are FIXED rates. You should probably pay attention during this next part.
This is an important juncture to ask yourself a valuable question: what if you charged for percentage of SEO earnings (say 5 or 10%) recurring monthly?

Let's say your SEO guy charges $1000/month on SEO to the business in question.
You'll only get $50-100 that first month.
But what if you focus on generating category 4 leads, which are usually strong?

You've got yourself $50-100/month/client.
Lets say you manage to gain ONLY TEN such clients, which last for a year.
That's $6,000-12,000, and that's where the money lies.

Keep in mind that this post assumes that your SEO guy is capable of performing a lovely, responsible job.

Happy sales!

If you trust your friend, suggest that you do this on a referral basis.

i.e. you get a percentage of any work generated - say 10-15%. Works for my old company!

Do it on a year 1 rev basis as well, paid when he/she gets paid