The usual argument is that price should come after the conversation. In practice, leaving it off means half your enquiries are from people who were never going to pay it, and you spend your evenings having the same disappointing phone call.
Putting a real number on — per session, per block, per month — costs you the wrong enquiries and barely touches the right ones. Trainers who do it consistently report fewer enquiries and more clients, which is the trade worth making.