I like the idea of separating the cost of making the font from the cost of licensing it (which could mean assignment of rights or not). If you did it this way, you would have at least two line items to your price. I'm sure there are other ways of being fair but every time someone has tried to explain one to me I got confused.
The reason I like this way is that a big part of why the client might think a price is "fair" or not is dependent on how they intend to use the font.
There really are clients who want to commission a font that has extremely low cost use (on our model that would be desktop only to generate rasterized images installed on under 20 CPUs) but that use is very important to them so they are comfortable paying for the labor, just not licensing they aren't gonna use. If I charged them what I would charge the client who wants to give the fonts to a large employee base, a large team of contractors, web embedding on a high traffic sites and embedding in apps with a lot of users there's just no way that client would hire us!
@James Puckett You're correct, of course, about complexity being something to avoid but you're wrong about what constitutes complexity. You're also probably wrong about what keeps people from reading EULAs.
People don't read EULAs because they have a preconceived notion of what legal documents are and that they are hard to read. Complexity doesn't keep people from reading EULAs because they largely don't get far enough to know if the document really is complex.
Complexity is amorphous clauses about "large volume" use or similar, rules with caveats or exceptions, hard to understand concepts, or disconnected ideas that require memorization.
A simple request to credit the font designer is superfluous but it doesn't add complexity. The problem with complexity is that it is a tool used by crooks and scoundrels to trick people. Everyone knows this and so even when those of us who mean well allow ourselves to be complex we sow mistrust. Trust is very important in a negotiation so it's best not to throw it away before one even starts. Since a lot of the people who read a font EULA are doing so as a first step in a license enforcement settlement the goal of a good EULA should be to make sure that person reads it and doesn't think it's complex and confusing.
I run an independent foundry and I can tell you that driving font sales requires a great deal of costly effort. Saying "sales is sales" is like the web developer who says "fonts are fonts" and chooses to use a system font because it saves processor speed.
The infrastructure alone for font sales (shopping cart, database, website) is costly and complicated to build and maintain. These things have to be built custom because of the nature of how we sell fonts. That alone is probably worth a 20-25% cut. Then there's the fact that users want to look for fonts in large libraries. I speak from experience when I say that it is much harder to drive sales to a small library. So, just being a part of a large library might be worth 5-10%. Then there's customer service, which can be considerable even with online sales - another 5-10%. Then the cost of advertising... it's very easy to fairly justify a 50% cut.
If you're still doubting my logic then ask yourself this: Are you willing to try to start your own foundry to publish and sell your own fonts? Maybe you've looked into it and know what that would cost. Even if you think that would ultimately be cheaper than using a reseller the opportunity cost associated with not being able to sell your fonts RIGHT NOW is an issue. If the answer is no then the conclusion is that the "fair" reseller cut is the one which results in the most sales for you.
Yes, FontSpring is a respected seller who charges only 30%. They do that by keeping licensing simple (only perpetual, lots of one size fits all pricing) with the idea that the money will be made on volume. That's a valid business model but very different from the Monotype model that dominates the rest of the industry. I can't say if one actually brings in more money because we don't work with any of them but I wouldn't be surprised if the foundry net is very similar in both models. Therefore, I'd suggest that you think not in terms of percent paid to the reseller but in terms of which business model you prefer (both in sensibility and in net to you).
You might be asking yourself why the CEO of a independent foundry that works with no resellers other than Adobe is making this full throated defense of the value of resellers. Because there really is a logic behind their existence and the amount they charge for their services. There's another set of logic for why we don't work with them but that has nothing to do with whether or not their fees are "fair"
I'd recommend being very careful to keep licensing and work separate - both in terms of invoice (separating each line item) and paperwork. This helps prevent confusion on the part of the client when they, inevitably, miss remember the agreement and innocently assume the new thing they are asking for is already covered.
In that case the question you asked would be a licensing add-on that would not effect a work contract.
If the client were to request a change to the scope of work (say they add a style to the deliverable) then that does affect the contract. In most cases where a contract is in place and you are changing a detail you'd do an addendum to reflect the change, not a whole new contract.
One, pricing for add-on licensing is rarely done in a vacuum. Foundries price web embedding, just to take one, based on as assessment of the use (usually traffic) as applied to pricing tiers. So, the only choice when pricing for "unlimited clients and unlimited use with no time frame" is to price at your highest possible prices for a very high number of clients and uses. And even then you might be wrong.
Two, unless they are required to report each client project to you then you how no way knowing if a given observed use is licensed. And I'm sure they don't want to report to you since, as Pablo pointed out, the whole point of this seems to be to remove all friction for them.
If I did agree to this I'd do it for a short period (one or two years) as a test. I'd require they keep careful records so we could review and reassess the pricing at the end of that term.