Unless specified, strategies do not have a gold cost. The announcment of most strategies actions do not trigger the "when paying a gold cost" part of gold production traits. It is not a gold cost of 0, gold cost just does not even exist. By the C.R. the Invest trait allows the player to "add to the cost of the action." But there is no cost to add to. At which point does the gold cost of the action transition into existence?
Gold Cost of Strategy Actions With Invests
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Unless specified, strategies do not have a gold cost. The announcment of most strategies actions do not trigger the "when paying a gold cost" part of gold production traits. It is not a gold cost of 0, gold cost just does not even exist. By the C.R. the Invest trait allows the player to "add to the cost of the action." But there is no cost to add to. At which point does the gold cost of the action transition into existence?
From the CR: An Invest cost on a Strategy may be added to the cost of any action the player is taking from the Strategy, at the point when that cost would be paid.
When you get to the "Pay costs" step of the action, you declare that you're using the Invest trait, and the cost gets added to the action's cost. If the action has no cost, the cost - like any other missing stat - is treated as 0, to which you add the Invest cost, which you may now pay for.