I am assuming the car does not start at all, and not just a hard-starting problem (hot or cold).
Have the mechanic determine if it is an ignition or fuel-related problem...
...you didn't mention what year your car is, but if it is ignition, I would take a stab at the ignition coil(s), but any one ignition component could cause a problem (cap rotor, etc).
...I would suspect the fuel pressure regulator if it is determined that the igntion is sound.
These are just tips to assist in troubleshooting and are by no means an expert diagnosis, or an excuse to resume throwing parts at the problem...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car