First things first ... the OP asked about a simple and secure way of having a "pay per click" service for lisp, not about the merits of such a system.
That being said, the solution is that you can have simple or secure, but both together does not seem plausible. Let me explain why.
Lisp as a platform for development is not secure. You can compile the code to VLX, but VLX can be decrypted or you can obfuscate it and while that makes it difficult to read it does not make it too difficult to hack. Other options include creating special encryption for the core of the application, but that requires building an encryption/decryption function that a user could easily hack and subsequently use to decrypt the application and circumvent the security.
It was previously mentioned that perhaps you could use a server to track the usages, however, what if you used a server to manage the lisp core application and used a similar approach like
this to retrieve the application and also use it to track usages. You wouldn't need to have the whole application on the server, only important parts that are critical to the operation of the program.
Now to the merits of such a system.
Some folks say that they would not use a "pay per click" application, and while that may be true, I think that in truth, they would in certain circumstances. Having a fixed cost system for unlimited access has its merits and is probably preferred in a high volume environment, but, for those environments where there is value in a "pay per click" usage scenario, it is a great solution.
All you need to do is look at the cell phone industry, and while it has migrated almost universally to the "unlimited access" model, there is still a thriving market for paying only for your usage. A clear example is my state issued cell phone. In the past, I used to ask the question: "In this modern time, why do we hear about government agencies back-charging employees for personal use of cell phones when an unlimited plan is so cheap and there is no additional charge regardless of usage?" The answer is (I found out recently) that most government phones are on a pay per unit schedule. My state bill hasn't exceeded $5 a month, but my personal bill is still $50 a month and I talk on my state phone more than I do my personal one!
The point is, a "per use" application makes sense in some cases, probably more often that most people would like to acknowledge ... but then people like the idea of paying a flat rate for something and not having to continually pay for it, even if it is more expensive. AutoCAD for example is nearly $2650 if you do the subscription, you pay each year ... for some folks that equates to a lot of "per drawing" expense. If Autodesk charged a reasonable "per drawing" fee instead of the $2650 per year, you can bet that people would jump on it, especially considering a subscription costs on average $100 per day, per employee. Suddenly a $10 or $20 per drawing fee doesn't seem so outrageous.
