I'll use a level 25 cape as an example. It takes 1275 fps to level and 1st place gives 295 fps. If you have friends with 80% arcs then if you pay 213 fps into your own cape 1 friend should be willing to donate 531 to your cape to lock 1st and the other should be willing to donate 266 to lock 2nd. With 265 fps remaining you may be better off using swap threads to finish but suppose instead you add 85 of your own fps, another friend should be willing to donate 90 fps to lock 3rd. With 90 fps remaining, if you add 36 fps another friend should be willing to donate 27 to lock 4th. With 27 fps remaining if you add 9 another friend should be willing to donate 9 to lock 5th. That leaves 9 for you to finish up.
Doing it this way you donated a total of 352 fps to gain an extra daily fp. If you also have an 80% arc additional daily fps are really worth around 2.5 so your payback is roughly 140 days. Of course this all depends on your arc and the arcs of your friends and guildies.