Your employees could quit when they get other offers from businesses that care about professional development. You will have higher turnover and institutional knowledge will constantly be leaving and having to be relearned. Onboarding new employees more often has a direct, measurable cost but is obviously different for every business and position.

Your employees could learn advantageous and/or competitive information from talking with people at the conference. They will bring this back to you and ask for a raise when it turns out it's a bonus for everyone, but that's what we want, right? Maybe it's important enough that you get to pitch the new thing to the CEO and you get a raise as well. Everybody wins.