I think you might be targeting more than 1 success on your skill challenge and actually that is possible to do ie to spend more.Right, maybe as an extreme you can 'temporarily sacrifice' your capability, like your troupe is decimated and must regroup, but you get an automatic success as they put up a stubborn rear-guard action to allow the refugees to escape. Later you can accept some sort of cost to undo that, a treasure parcel, a personal debt, etc.
You could even, in the right situation, make it an 'ultimate sacrifice', the 300 are wiped out to a man, but the great evil army is turned back at the pass! They're gone, you won't get them back, but of course you've created the basis for a new legend.
The prices I am setting up are assuming a single SC success and flavor wise I assume most of the time there is a SANE call for a retreat and avoiding the slaughter thing (remember the word decimated meant lost 1 in 10) . In some sense it is a part of the deal of having the practice. I presume a somewhat nebulous membership you are in theory always loosing some and always inpiring new ones to take their place
Other actions in the skill challenge can also affect the in story impact on the troops, finding vulnerabilities of the enemy via application of skill and the like. Distractions and the like so the bad guy doesnt follow through on them. An over all failed skill challenge might affect them to the negative.
Your troops are a resource largely based on reputation and recovery so the "wont get them back" is analogous to you no longer have the practice anymore.
Perhaps with the price to recover based on the level you purchase it for good measure.