[Occasionally6]

The gain won't be dramatic but will be noticeable. Something around 5% (10% max) would be about right. You will get more back than you would by fitting it to an engine that never had it because the ECU will be mapped on the basis of it working (unless the mapping has been changed to suit the disabled EGR).

The air:fuel ratio will have been pulled back to stoichiometric by the ECU with feedback from the O2 sensor even with a speed-density (MAP sensor) system so that won't have any effect (unless the provision for correction was exceeded). However, the ignition timing won't have changed and will currently be overly advanced (mapped for EGR) without the EGR present. That's why you'll get more back than fitting one from scratch.

Surely the parts required are going to be very cheap from the junkyard. The valves (EGR valves in general, not specifically those used in the Neon) do get blocked up with combustion residue so you'll have to clean it, but they're pretty reliable otherwise.

With engine management in place the EGR is not usually (although it may be in later cars) active at WOT so it doesn't cost any power. (Other than the small weight reduction, it was really stupid to remove it.)