Might not necessarily cost you much of anything depending upon how you wish to approach it. All you need is to write the NBIC scope into your manual and have your company surveyed to that scope.
Your a boiler company so I assume you already have an AI. Take advantage of that.
Your additional cost would be if your AI charged you for time to consult or if you brought in a consultant to advise on what to include in your Manual.
Another cost would be the survey itself if you didn't want to wait until your boiler audit came around. Call ASME and ask them.
There might be an additional cost to the survey with the added scope. 
Get yourself a copy of NB-57 and review it for what you need in your Manual. You can download this thing for free at the NB website.
This is the document the survey team uses to evaluate your system. If you comply with it you pretty much have everything.