The Skywatcher and Celestron 8-24 zoom eyepieces are decent - I wouldn't pay more than $100 for one and I don't recall how they fare at f/5, probably good enough though. The field of view is narrow.
You might want to consider putting the money towards the Explore Scientific 82 degree eyepieces instead, many of which are on sale for $100 each.
http://www.optcorp.com/ProductList.a...-718-1511-1512