Replace prorated warranteed(40%) cast iron heat exchanger or replace 18 yr. old unit.

We have a n 18 yr. old burnham series three. The heat echanger is non useable. Do I go with a new boiler or all the stressors of getting the exchanger replaced. Looking at a lennox gwb3-e replacement. Just trying to see if the difference in price would be huge, or minimal. Having a hard time getting a straight answer from all but one dealer.