It's hard to say exactly, but here is what I'd do if it's a SaaS app.
Try to determine their customer lifetime value. That is, how much is each customer worth from day one to cancellation. Then apply a formula based on the amount of customers you think will be created.
If their average monthly price is $89 and on average, a customer is a paying customer for 2 years, then the lifetime value is $89 X 24 months = $2136.
Now, estimate how many of your users will convert to customers. 8,000 (downloads per month) X .01 = 80 new customers.
So, at a 1% conversion rate, they will earn 80 x $2136 = $170,880
Or at a .05% conversion rate, they will earn 40 x $2136 = $85,440
Now, these are all estimates. But, with a little research, you can probably get in the ballpark.
Next, what is their current customer acquisition cost? Again, use your best judgement. Look at Google Adwords and try to estimate what comparable traffic would cost them.
If, on average, they spend $600 to acquire a new customer, you should charge accordingly.
So, how much should you charge? Well, if you're creating $170,880 of customer value then you should charge accordingly.
Keep in mind, these companies HAVE TO pay for their users. They are looking at your project as a marketing opportunity. Charge accordingly :)