You can go direct from Client to OutsideCosts ... depending on the speicfics of the relationships.
Try it where you just skip a level... then go deeper. Some relationships set to add new records can add the intermediary records too.
It sounds like you would benefit from a hierarchal display for user selection. You can use virtual list or portal or list records themselves (from the lowest level, constraining and extending) to achieve it. I should think the easiest to implement for someone newer to FM would be using portal.
Excelisys has just released such a demo http://fmforums.com/forum/topic/88032-hierarchylite-filemaker-pro-12-tips-and-tricks-demo/
While there, here are a few more incredible demos on this thread as well by our infamous Michael Horak and John Ahn.