How do we determine the value of a patent?

It is impossible, or at least very difficult, to put a price on such patents. You would only know their real value when you would be selling or licensing them.
Having said that, there are various methods to calculate the 'value' of a patent, but all of these methods will be providing different answers, since there is no one, fixed way of determining the value.
In your situation probably the best method is to look at the historical costs that have been made to get the patents (or patent applications) at the stage where they are now. This then should be considered the expenses that your co-founder did have to make and this 'money' is brought to the cooperation.

Hi,

I agree with Bart. That said, I'm not sure if it will help, but there are companies that keep up with patent royalty rates per technology/ industry and/or can help "value" the patents. It matters what technology area. For example, patents that read on standards are subject to FRAND, and are usually no higher than 3%, while in other technology areas they can be as high as 10%. Try searching for companies like this (I'm not recommending them - just an example): http://www.royaltyrange.com. http://www.pellegrinoandassociates.com

Hope this helps,
Mary