How to price a software "per project" relatively to its "per seat" price?

By targeting multiple segments, we have to price our software according to different schemes. We are currently thinking about defining a "per project" license.

We are doing the following computation:
- the software usually pay for itself over 4 projects
- users would not have more than 4 people working on the same project at the same time.

So our "per project" price would be "per seat price" / 4 (projects) * 4 (people).
Basically we would have a "per project" price = to our "pear seat price".

How do you price your software per project ?
Do you use the same kind of computation ?
Do you compute a % of typical project budget ?

I have a bias toward pricing that is simple to explain, simple to understand, and motivates the customer to use the product.

I understand that there are project management SaaS solutions which offer their pricing on a project-by-project with a metrix of users, owners, and how many times you send out flowers. I choose not to use those products. To me it would be like MS Word charging me per docuement, or Dropbox per synch file, or itunes per (oh, wait, never mind . . . )

I don't want to have to pull out my calculator every time I consider adding another project onto my project management platform. I don't want to have to add it as a cost of goods to my proposals.

I want a fully integrated solution that I can integrate with my CRM, my accounting, reporting, and customer service that allows me to add and delete projects, add and delete users, add vendors to multiple projects with one dashboard -- which allows me to test the service for free, pay for it as a basic level as I ensure it will integrate with our workflow -- and then upgrade to an enterprise version when it has become essential.

I am not saying that I am like every user, but based on where other pricing models are with other software -- my assumption is that they must be a good number of other users just like me.

But then -- I might not be your target market, so what I think of your pricing might be irrelevant -- what is "neat" about pricing is that the model should match the needs and desires of your target market.