My 300SE radiator was around $200 but obviously a different car and radiator.

And the R&R procedures are likely very different.

If you do try the neck reinforcement approach, assess the condition of the plastic carefully as the crack may be indicative of rotting of the plastic which would make any repair strictly temporary.

Also make sure your choice of adhesive is rated for the expected operating temperature and make sure both the neck and the reinforcement piece are clean, dry and well roughened to give the adhesive something to bond to.