Ahhh, this is an age-old question! I feel your pain My approach has been to buy the figures individually, over time, when I saw one that was in good condition. Could I have saved money buying a lot? Sure...but then I probably would have felt badly about the figures...POP gals in lots tend to be missing clothing and have super matted hair. So if you buy a lot, read up on other fan's techniques for detangling hair and washing figures, and be prepared to put some time into "restoring" your collection. You also run the risk of getting a bunch of figures without accessories. Then, you'll end up hunting for Frosta's wand or something, and auctions for single accessories tend to be pricey. Ever seen auctions on pieces of the Crystal Castle? Yeah, those can get expensive.
So, my bottom line is that it's like buying anything else - a house, a car, clothing - in that if you want it to be in perfect condition, you'll end up paying for it...but buying it in used condition might mean you don't get everything you want and have to work a bit to fix it up.
If you're just looking to own the figures, and aren't a super huge fan, go for a lot because you will save on shipping at the least. But consider looking for the figures you really love in better condition and buying them on their own.
Hope that helps, and be sure to check out the POP Marketplace here - you never know who will be selling what, and fans definitely take care of these toys