By the way:

I often use NULLIF when 3rd party application use MAGIC VALUES instead of NULL.

e.g. 1799-12-31 stands for NULL DATETIME.

Here's a generic example to get an actual price:

SELECT cArticle,mPrice FROM tbPrices

WHERE GETDATE() BETWEEN dtStart AND ISNULL(NULLIF(dtEnd,'1799-12-31'),GETDATE())