Asset valuation is a rich pre-existing field that doesn't need to be reinvented. The real cash-equivalent value of this app is, of course, the market value you could get trying to sell it, but that just offloads the value estimate to the potential buyer(s). The computation is extremely straightforward for a perpetuity, which is where the estimate here is coming from. At a 4% annual discount rate, a perpetuity that pays $500 a month forever has a net present value of $150,000.

However, as others have hinted at, assuming this app will continue to generate $500 a month forever at roughly the same risk premium as a perpetuity from any remotely reputable seller is dubious at best. Major banks and insurers that have existed for several centuries have histories of selling instruments like these that actually have paid out their promised coupon value for many decades and even centuries in some cases. In contrast, the Apple App Store has existed for almost 14 years. The 5/3/1 strength program has existed for 13 years.

What are the chances this app can generate the same revenue for the next 30 years? You can't rely on just keeping the existing users due to the way lifters tend to program-hop eventually. Anyone doing 5/3/1 today is unlikely to be doing it in 30 years. Then you have to factor in the possibility of competition. I'm kind of surprised the author of the 5/3/1 books has never bothered to commission an app and apparently only distributes official spreadsheets for generating a program template and workout log. If he ever does, any unofficial app seems unlikely to survive.

Not to take away from the work. I think this is the right idea. The developer wanted something that didn't exist and made it. Whether or not you ever acquire a single customer other than yourself, I think that is worth doing. But if anyone in this comment thread truly thinks it is worth $150,000, offer him $150,000 and I'm sure he'll be glad to sell.