Creating SEO goals and constantly measuring performance and achievements will enable every business to analyse their ROI.
It is very true that it is tricky to measure the value of using an SEO agency. We all can give you positions of keywords where you were ranked "73" and now you are ranked "3". The real value is of course increased sales as a result of new customers finding you on a search engine and that increased sales figure far outstrips the cost of you getting that ranking.
What I would say however is a professional SEO service helps you identify opportunities and focusses you on what is important to promote. It teases out from within your organisation what makes you an expert; what makes your product different;  why potential clients and customers should have that conversation with you.
Having that focus is definitely worth the expense as ultimately a better ranking is as a result of you demonstrating usefulness, trustworthiness and authoritativeness to any potential client whether they have arrived by search or not.