In a tech company it will most likely be significantly lower, but it depends what kind of product manager and engineer we are comparing. If you have an MBA from a top school and relevant experience you can get 30-40% more than your colleague who is given a chance to learn on the job. The most important factor for you is whether your technical skills are actually needed for this product manager role or not. Are you uniquely fit for this particular role?
I would speculate the best you can do is to negotiate to keep your current salary. Chances are the hiring manager doesn't know how much you make and might not have the budget to give you the job at your current rate. If compensation is important to you, consult with HR before making your intentions known to too many people.