The answer to your question is dependent upon what you plan on powering with 120 volts. There are many inverters on the market that are sized to different loads and the price and 12 volt requirements change with each.
On our Ranger 25, our 120 volt needs are very simple. We charge our cell phones, charge AA batteries, and charge the laptop. Everything else is 12 volt. Our needs only required an inverter that could supply about 8 amps of 120 volt current. The inverter we have is a small unit, that plugs into a cigarette lighter and cost about $35. We are really happy with it.
We chose not to expand into the world of 120 volts with appliances and entertainment media. This is kept our world much simpler, less to buy and maintain, and less maintenance issues. If you are thinking of using your home appliances, TV, etc on the boat, you may need a considerable sized inverter and then the battery bank size to handle it. All of this is quite possible and many boaters have done it. With the right size of budget, you can probably accomplish anything.