Patent costs are ongoing and somewhat variable, so be sure you each have a clear understanding of the costs so you can share them fairly as time goes on. The initial preparation and filing is significant, but so are maintenance fees, foreign filing costs, etc.

The most common pitfall I see is that both people don't want to pay for later stage work that is required and the person not paying still wants their 50% of the patent profits. Be clear as to what happens in case of bankruptcy, non payment, etc.