I would like to know the accuracy in degrees (not feet) of the latitude and longitude coordinates that the GPS NÜVI 50 gives for a point.
There really isn't a simple answer to this. Primarily because you cannot simply convert a distance in feet to 'degrees' like you could feet to meters. The distance between degrees of latitude are generally constant across the globe but the distance between degrees of longitude varies according to latitude since they converge at the poles.
If you want to convert the distance of a degree of longitude at a given latitude there are a variety of webpages that will do that for you. Here's one: http://www.csgnetwork.com/degreelenllavcalc.html
So, on a good day with your Nuvi, you're looking at around 15 feet of accuracy. Which, at 45°N comes out to approximately 4.12x10^-5 degrees of latitude and 5.79x10^-5 degrees of longitude. At the equator, that would be 4.13x10^-5 degres of latitude and 4.11x10^-5 degrees of longitude. Provided of course, that I have done my math correctly - which I cannot guarantee.