Why create a UDF for something that trivial? You could just write the query without it like this:
SELECT t.CustomerID, t.CustomerName, t.MinimumOrderValue, mo.OrderValue
FROM dbo.CustomerThresholds t
CROSS APPLY (
SELECT o.OrderValue
FROM dbo.CustomerOrders o
WHERE o.CustomerID = t.CustomerID
AND o.OrderValue > t.MinimumOrderValue) mo
WHERE mo.OrderValue > t.MinimumOrderValue;
Sure, use a UDF if it's complex and you want to reuse the logic, but that logic is trivial.