With less than 12,500 listings, it does not make sense to pay for an "Anchor" store.
http://pages.ebay.com/help/sell/storefees.html
The difference between a Premium store and a Basic store is $34 month. The difference in fixed price listings is $0.15 per listing. So one needs more than 226 monthly fixed price listings to justify a Premium store.
Listing fees for auction style listings are not affected by the type of store.