When you do your cost comparison you forgot to factor in the cost to move it, seeing as thats just as much work as it is to install a new one your difference is the cost of the boiler. Anyone who pays to move a boiler over 10yrs old is in my opinion a bit daft, but go for it, and good luck finding someone willing to do the work, I always refuse these unless the customer can source a new flue, which will be no then