The value of 100 gold does decrease rapidly as you progress, so it's not too big of a waste if you use it. However, in most cases, it is unnecessary. The only time you should consider using it is if your units are weak against the sector's units, as then 1 bar can mean the difference between the death of an enemy unit or the death of your own unit. But if they are not weak against them, then there is no point in infiltrating, and the 100 gold is better saved for something else, like additional forge points.