NavigX won't drain your iPhone's battery any faster than any other app that uses its GPS services.  In fact, NavigX has been designed to conserve power when it can.

Under iOS 4, NavigX can use the iPhone's new multi-tasking features and, when you switch out of NavigX to another app, NavigX automatically scales down its position updating so that it only reacts to any major changes in your position. When you switch back to NavigX, it restarts its finer position updating so you can continue on with accurate position reporting.

The largest drain on your battery will be if you walk with NavigX on the Navigate screen permanently as all the system idle timers are switched off to ensure you don't have to keep reactivating the screen.