The only reason to be enrolled in a 141 course is if you intend to graduate and get a pilot certificate under 141. If the school has examining authority like american flyers this means you don't have to take a flight test once you graduate.
If you are purely building time then you wouldn't want to do it 141 because there are restrictions on solo flying in a lot of 141 programs. Usually there is more dual and less solo requirements for a lot of licenses because you can finish the course in less hours than someone in a 61 training environment.
The cheapest way would be to find a flight club not a flight school and join the club. For example a club in my area (chicago) have C172's for less then $60/hr including fuel. At a 141 school you will always pay more than this.