The best way to determine current value is to look at SOLD prices on eBay.  Login to your eBay account and then enter the name of the item you want to sell/purchase ("A1-A4 D&D modules lot/set").  You'll see listing of for-sale items.  Then look on the left-hand side of the screen and click on the "sold" button/toggle and click on that.  You will be able to see all the listings that sold recently.  Remember that prices vary according to many factors, the primary one being condition.  The reputation of a seller can affect the sales price, as can the quality of pictures provided, and also the price charged for shipping, and more.  It also helps to modify the name of the search (try "adventures" instead of module, for example).  Finally, this site has a grading guideline page which can be helpful, although there are people on this site who feel like it's not exactly accurate.  Still, it's more help than not.  :)