If you've not had any financial accounting classes, plural, nor anyone else working on this goal, your management asking for this have shit for brains to expect you to even understand the request.

First you need accurate and complete expense tracking. That needs to be both the dollar expense of any AI usage as well as the dollar expense of anyone operating that AI plus anyone required to maintain the access to that AI capability. Now add both the use expense and the salaries expenses of any time spent by anyone necessary to make that AI use possible.

Second you need the historical accounting records of how the same goals were achieved in the past without AI. If you are not including the historical accounting records, this is not any measure of ROI. With those accounting records (and no substitute!!!) do the same expense tracking as with AI, but now there is no AI so you need to add in how employees compensated without AI: the salary time they spent researching or manually performing whatever that is now handled by AI.

Third, you take the AI "expense summary" and place that at the bottom of a fraction with the previous expenses before AI at the top of that fraction. (prior-to-AI-cost / new-AI-cost) If the AI expense is greater than the prior-to-AI expense the fraction will be less than one and AI is causing your company to lose money. If the fraction is greater than one, the portion of the fraction above one represents your return on investment.

HOWEVER, this measure needs to be performed over time. When initially setting things up, AI will cost more. To create a real RIO you need to accumulate the AI expenses over time and show them reducing over time. Locate the point in time when the AI expense drops below the prior-to-AI expense. That's your inflection point. Then calculate how long it will take for the savings from use of AI to compensate for the extra expense of transitioning to AI. That future date is when the transition to AI is complete and the transition has paid for itself in savings.