Backing Shortest Priced Selection

I have a trigger working fine that chooses runners in non handicap races matching certain criteria. Occasionally it turns up more than one runner in a race, in this situation I would like it to just back the shortest priced runner. Is there a simple way to achieve this? I have Selection set to First Matching Selection.
Cheers
Mike

You could try adding:

and Selection's Back price is the minimum (of all)
and Selections  Back matched is equal to 0

If conditions have already been applied then: and Selections Back price is the minimum   - without the (of all)

Hi
and as a safety net after rubold's condition you could add this as the last condition:
and selections trigger expression matching_number is equal to 1

in other words if there is more than one matching selection, no bet