How much does the app cost the ecg app for Apple Watch

There is no extra cost for ECG. The hardware/software support is part of the Apple Watch Series 4.

However, each country's health agency must approve ECG us in their country before Apple is allowed to enable it.

Check if your country is listed as having approved the ECG capabilities

Apple Watch ECG countries

https://www.apple.com/watchos/feature-availability/#apps-ecg