Adrien,
1.) Did you consider to try asking an other constructors. If you want I can give you some alternetives (mainly East-Flanders, others I will need some digging) where you can ask for quotations.
2.) If you want to switch to pre-wired systems, you also need to consider the extra cost of spares for that system, and the future availability of this type, not all systems are downwards compatible.
3.) In our company we use mainly the interface modules of Phoenix contacts, which have the advantage that they have special products if you revamp your PLC (Change Siemens S5 to Siemens S7).
If you have more question, shoot!
Regards,
Robby