Product pricing - base price + user price

I have a new install of 1.38a and have a pricing question.

We are going to be offering products with a base price and then $xx per user:

Base Price: $100
Price per user $10

I've tried to do this with a product and attributes but the attributes won't do the math. ie: 5 users = $50 on top of base price.

I've tried pricing the product at $10 and adding a onetime cost of $100. It doesn't have the look that I want as the price is listed as $10 and it feels like i'm hiding the base cost.

Is there a simple way to do this sort of pricing? I though I would ask before I start getting into the php files.

Thanks for any insight you can offer.

Do you mean like software licence 'seats' ?

Yep that's the sort of pricing structure i'm looking for.

Assuming there are only certain numbers of 'seats' that can be purchased ( that is, I can only buy 5, 10, 50, 100 etc., and not 27), attributes would be the way to go.

You would set up the product with a base price and then each attribute would add a fixed amount - 5 seats = base price + $50.

If you do want to allow me to buy 27 seats, you would need to set up the attribute with a price of $10 and I would add 27 to the Add to Cart input box.

We want to allow the customer to pick any number so i've been attempting to price the product by attributes with no success.

The product is set to be priced by attribute and I have the attribute setup with Price + 10. In the product listing it's showing up as $10 but when I type in a value and add to cart the price is $0

The option name is set to readonly and the only flag that is true is for adding the attribute price to the base price.