I would like to know first if DTW is the most convenient International Airport nearby Auburn Hills, MI? And what is the cheapest way to get a ride from the Airport to Auburn Hills?
Here is a map from gcmap.com showing all the airports near your target location that fly to London, UK. I chose that since almost anywhere will fly to London somehow. X marks your target location roughly. Which airport is best depends on where you are travelling from. There may be one or more stops from most of these airports, with DTW more likely to have a non-stop (or actually be on the route to/from the other airports).
I entered a random date searching for flights from FRA to DTW,XEC,YQG (YQG has complications since it's in Canada) and it does seem that DTW is the only reasonable option.
Looking on http://uberestimate.com, a typical UberX price from DTW to Auburn Hills is stated as $45-$58.
Google maps transport links to http://smartbus.org. However it's looking like a 3-3.5 hour journey to get to Auburn Hills rather than a 45 minute taxi ride. Are you interested in that?