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One of the big expenses is the new DC electrical subsystem that needs to be installed.  You need to run new DC wiring and appropriate circuit protection from the batteries, through the new inverter switch at the panel, to the inverter.  The reason you need new cables is that even the smallest inverter/charger you are likely to install draws much more DC current than your existing charger supplies to the batteries.  The cables you will need depend on the size of the inverter and the distance from the batteries, but they are likely to be 'huge'.  Huge cables are expensive to buy/make, and time consuming to properly route and support.

Then there is the task of properly tying it into the AC system.  This can be complicated depending on the existing AC system.  Is the existing AC system just a single phase 120v shore power input?  Or, perhaps it is multiple shore power inputs including 240v split phase, 120v single phase, and a 240v split phase generator?  What load(s) do you want the inverter to be able to run?  A single AC load, select AC loads, or all AC loads?  The answer to these questions can have a significant impact on installation complexity.

Then there is the matter of supplying adequate energy to the inverter.  Depending on what you're going to do with the inverter, it is quite likely that you will need to increase the size (and possibly the chemistry) of your battery bank.  That said, a properly designed and installed inverter system is great to have on a boat.