Which kind of aircraft have the lowest operating cost?
150 Miles is perfectly reasonable distance in most small planes and one passenger is a fairly easy load. There are a lot of options out there but arguable the cheapest would either be the Cessna 150/Cessna 172 Or the Cherokee 140/160/180. The factors that keep the price very low, 


Fixed gear: less to service and inspect during an anual and less to go wrong in terms of over all maintenance. You never have to worry about the proverbial "gear up" landing. 
Some of the most produced air frames in history by companies that still exist so parts are abundant and not all that hard to come buy when compared to some of the more obscure air frames out there.
Very common engines, decent on fuel burn rates and easy to have serviced, inspected, and generally considered reliable. 
Very practical useful loads for carrying one other person and luggage. 
Common and for sale in most places, would be easy to find a decent air frame.
Common enough you may be able to find a partner to go in on it and help reduce costs. 
Lots of mechanics out there who will happily work on these planes, and are quite knowledgeable on them as well.