How to calculate?

The least you can expect from it is $0.00. There are way to many variables - the sites set up, the the niche, your pricing, your competition, the sales copy, the type of site, your abilities, where you advertise, etc - to even take a guess.

Are they qualified prospects? Or just random junk traffic?

What are you measuring?

Unqualified Leads >>> Qualified Leads >>> Sales

What will your site do to ensure these three elements of a successful funnel are included?

Throwing crap at a wall and hoping something sticks is not "To Manage We Must Measure."

Set conservative targets, and then achieve them.

Then raise them.

You ARE only guessing at the start. It's an estimation. What do you think is reasonable and conservative for those three variables I listed above?

I would do it the other way around. How much money do I want to make? How many sales does that mean? How many qualified leads do I need in that case? And therefore how many unqualified leads?

Total revenue desired / Average sale price = # of sales needed (I would probably add 25-50%)

You want $2000 a month?

Say an average sale price of $50...means you need 40 sales.

And you don't want them right up to the wire, Day 30, either do you. No, you want to pass that point with some days to go in the month so you have some breathing room. So let's make it 50 sales.

How many qualified leads do you think you need to hit 50 sales?

Conservatively?

My guess, and it's a GUESS, is 20 interested people who came with the intention of buying this Wobbledygook 3000 from someone, maybe you, in the near future...

50 X 20 = 1000 qualified leads!

And what have you just learned?

That if all goes according to plan, you DON'T have enough traffic to hit your money target!

Now how many unqualified leads do you need to get those 1000 qualified leads?

Say 10. Out of every 10 goofballs who accidentally visit your site, 1 is interested in that Warbledy-I-Can't-Even-Remember-How-I-Spelled-It.

10 X 1000 = 10000 unqualified leads.

Now you know what realistic traffic level you require.

"I want to make as much as I can" is the fool's errand and the newbie's poorhouse.

Set targets. Adjust. Achieve them.

No battle plan survives contact with the enemy.

Same here.

You will find your estimates may reflect reality or be way off.

But now you know where to look.

If you dump 10000 leads into the funnel, and only 200 are qualified, now you know you have a traffic quality problem.

If you put 10000 unqualified leads in, get 1000 qualified, and sigh because only 20 people bought, now you know you have a conversion problem.

Sure beats throwing crap at the wall and hoping something sticks, don't it.

Honestly since you don't know the true answer to this question (because it hasn't happened yet), you should know that your focus should be on the "backend marketing" side of thing. How much money do you think your average customer will spend with you over the lifetime of you having them on your customer list? Once you have this answer, you can then determine how much money you're able to lose when getting a new customer (some people WILLINGLY are able to go negative) - because you can easily recoup your investment soon due to backend selling, and the average value of your customer base.

When you take the totality of results everyone has achieved, you can come up with a %. But that percentage varies widely.

There are a few factors that sways this figure and that is:
traffic > offer
title
salespage
product & price
sales funnel

To see how valid this point is, go to the forums and check out amount of replies and you will see there is a vast difference between amount of views and replies, and likewise visitors and conversions.

Some food for thought with targeting. People walking into a grocery store are 99% likely to buy. Someone walking into a computer shop might only be 40% likely to buy. Those are not accurate figures but with some thought we can see those metrics likely hold up.

Finding those base figures though, I suggest doing a trial run to get your base figures. Nothing like getting a practical baseline.

My recommendation is to have contingency plans instead of wishful thinking. Start off with calculations at 1%, then 5%, then 10%. Then implement your site and then seek to improve once you have your base figures and you do that with looking at the sections above and improving them. So, in essence, don't focus on that's the number, plan for the worst and work your way to the best.