1998 Mercury Sable car wont start

Well something seems to be either draining my battery or not sure I had to replace battery in dec of 2012 battery was good for amonth then I had to jump it every time I wanted to go so reurned battery got new and car started right up basically the same problem would happen about 3 weeks into new battery I wouyld exchange it then the last time we figured that we would not keep the battery hooked up and I can just hook and take off conncections everytime I want to go or it sits for a bit well that worked for like a week and now the car wont start at all even with a jump alternantor tested ok not sure what else it could be but mechanic wants to charge between 40-100 depending on the issue of the car just to LOOK at it and diagnose the problem please help me so I dont get taken advantage of