There are many vendor makes the 78M05 at different price, what makes them different?
Other than the package material, the most critical thing is the silicon design, in other words, how many active transistors are used. 

Here is what I have found:
1. OnSemi, MC78M05BDTRKG, 28 active transistors.
2. Diodes, AS78L05RTR-E1, 24 active transistors. 0.1A, not a 78M05 device, but similiar.
3. Rohm BA78M05FP,  18 active transistors.
4. ST, L78M05ABDT-TR, 17 active transistors,
5. UTC 78D05AL, 78D05LL, 78D05L, 15 active transistors,

The following are the schematics posted on their data sheets:
1. OnSemi, MC78M05BDTRKG,contains 28 active transistors.

2. The Closest Competitor: Diodes, AS78L05RTR-E1, 24 active transistors:

4.The third closest competitor: ST L78M05ABDT-TR, 17 Transistors

5.Other competitors uses 15 or less transistors: UTC, 78D05AL, 78D05LL, 78D05L, 15 transistors,

涉及到电源, 一定要用好的零件, 电源一坏, 故障指数级增长.

还是挺喜欢楼主的这篇对比.  

是不是可以理解为成本跟晶体管数有关呢?  这就不得而知了.^_^