It is unfortunate that the perception is this is a common practice of contractors, but difficult to argue when your neighbor slams you with a bill like that.Some neighbor you have.
I personally never have and never will perform work that is not clearly define of what I will provide and what the cost is, and agreed to, in writing. Obviously, not doing this first is a a bad business practice. I do not know any contractor that works that way.
But, once work is done and not paid, and I have had this done many times, with all the proper documents and agreements in place, I have absolutely no rights to take your home. I can even be arrested for trespassing if I come and visit to collect my money. I can place a mechanic's lien a debtors property, which is fairly useless, until the property is sold, and the lien is found in the town records. Otherwise there is no effect on a debtor with a mechanics lien attached.
Without knowing all the exact details,such as length of wire and conduit, size panel, labor costs in your area, we cannot speculate as to the cost the job should have been. THe $700.00 seems reasonable for the pool, $3300.00 seems excessive for a 100 amp subpanel and feeder, unless the line was very long.
There are several directions you can go with this, all depends on your past relationship with neighbor, and where you want to see it in the future.
If your willing to pay what is fair, perhaps you can call out another contractor. Pay him a service call, and ask them to look at the job and provide a detailed estimate, based on wholesale costs of the materials and published labor units for the installed materials, plus reasonable markup for overhead, profit, taxes, and permit fees.
Has this job been permitted and inspected? Does your city or state have a consumer agency that regulates contractors? Are there any laws in place that prevent this from occurring and penalties for unscrupulous contactors?
You can either fall over and pay him what he wants, or you can only pay hin for the $700 and what you think is fair for the panel, and let him sue you for the remaining balance, and with getting the other agencies and laws, and a fair accurate estimate behind you, you may be able to bring his terrible business practice to light, and get some relief from what appears to be a gouging bill.
Let this be a lesson to anyone that hires any contractor to have an estimate done with a firm price for a detailed scope of work , to include all costs of labor, materials, taxes, permit fees, and be sure the contractor has the proper amount and types of insurances in place before any work is done.
Sure am sorry you have this to deal with, and that there are still lousy contractors out there doing this nonsense.