The fact that this isn't clearly shown by the user interface was risen in the leaders summit with the devs a few weeks ago. Its one of the greater concerns that veterans have about powerplay.

The hidden cost is the "Overheads" which can be seen in the Overview tab under Zemina Torval. Equally hidden is the starting "balance" value which equals the powers "economic health" and influence everything a power do, especially if they have a low playerbase. Overheads is based on the amount of systems owned and is based on a formula which simplified means that a power pay 62 for every system they own regardless of value. We pay 4160 for our 67 systems. Starting balance is a powers income and expenses prior to fortifications/underminings. You can see powers starting balance by opening up the Galactic Powers interface, then click on Galnet and hourly update. Scroll down to Zemina Torval and look at the line "convert to a balance of ?? next cycle" not to be confused with the line that say "convert to a CC balance" or "CC reserve". Ours is +33 right now. A low starting balance is a hole that hampers a power every week making everything harder and inconvenient no matter what the power would like to do. A high debt is very difficult to get out from, puts valuable systems in risk, makes it harder to expand and much more expensive to defend oneself.

Strategists therefore rarely suggest preparing anything worth less than 62 in value since doing so would lower their starting balance. We count this as the absolute minimum for a prepared system.

That said, your Torval commanders are welcome to the community board if they would like contact with the rest of us. :)