It all depends, on what you sale,how much inventory you have, etc etc etc.
personally I love having a store but it isn't for everyone.
I would suggest doing research, the ebay store board is a great place to start. Alot of very helpful people, and great advice.http://forums.ebay.com/db2/forum.jspa?forumID=21&anticache=1145589448021
forgive the not so happy threads, since ebay pulled store listings from the search its been a bit sour over there, but there is still alot of good threads and helpful people.
Here is the search results from people asking the same question as you and the replieshttp://forums.ebay.com/search.jspa?objID=f21&q=Opening+a+store