You can enter the price for the flex track article in the Part List and have the app calculate the total.
Adding up the combined track length for a specific flex track article can be done (assuming all layers in question are unlocked and visible) by selecting one of the flex tracks, then apply Select > Similar Elements and refer to the section length field in the Info panel in the sidebar to the right (Element Tab). The text label is also selectable and copyable as text.
Note that you would need to have access to an infinite flex track to just use the combined length as it doesn't take scrap/clippings into account.
The issue is explained in more detail in the "Flex track calculator for cut of rest bits" feature request:
https://railmodeller.uservoice.com/foru ... -rest-bits
It's on our to-do list for an upcoming update.
Hope this helps,