Check out "Portals" in the online help. I'd suggest doing this with at least two tables and a portal to list the costs as individual records. This will be a much simpler and more flexible approach. WIth a portal, your "best cost" and associated fields of information can be set to automatically sort to the top of the list if so desired.
I noticed you mentioned an IPhone. Note that it takes quite a bit more work to publish FMP in a manor that will work for this use. You might want to research a product called FMTouch.