I am trying to build a case/if statement in a JOIN ON clause.
JOIN ON
LEFT JOIN [CTSTRC] [Statuses] ON RIGHT([Statuses].[STRID], 3) = [CTE].[F61]
You're looking for the ISNUMERIC function (I believe it was introduced in SQL 2005):
LEFT JOIN [CTSTRC] [Statuses] ON (CASE ISNUMERIC(RIGHT([Statuses].[STRID], 3)) WHEN 0 THEN 0 ELSE CAST(RIGHT([Statuses].[STRID], 3) AS INT) END) = [CTE].[F61]