if people are already paying that much, they wouldn't have a problem paying a premium for a tool that lets them get more out of their workoutsYou have to remember to explain how they gain from using your product in monetary terms. In exchange for $150/month in gym fees, I get to use their equipment, showers, electricity, trainers, cleaning crew, etc. When I use your app, what do I get? Maybe your app saves me 30 minutes three times a week vs using a diary. At my hourly rate, $30 is a bargain.
When I was doing freelance web design, you wouldn't believe how many businesses were spending $400/mo on paper YellowPages ads, but couldn't see the value in spending $500 + $50/mo for a website. When I explained that they could use just half of that monthly spend on SEO and PPC ads, and have better conversion tracking, they saw the appeal. When I explained that an online scheduler could save them $1600/mo over having a full time receptionist, they really saw the appeal. You have to frame it in terms of cost savings/increased profit/saved time, etc.