Divide your stack up into categories:
- High Premium/Numismatic
- Standard Premium
- Spot/Bargains
Calculate your average buy price for the items in each category.
If you've been buying a lot of high premium items like Lunars, it will make your average buy price look crap because they'll skew the numbers much higher than spot but you probably would have done pretty well on increased numismatic value which should increase the actual "worth" of your stack.
Obviously getting your average down as close to spot is desirable, but perhaps a better way of measuring it is to ask "How much could I get for all this if I needed to sell it next week?"