I'm only speaking regarding the MVS price guide.
When I first bought MVS carts, I didn't know the difference between the low and high price on the guide. I then decided to go to kits and used the high price as the guide for kits, but it still didn't seem accurate. I then emailed Shawn and he updated the MVS price guide to now reflect the fact that KITS are worth more than the high price, which is true for the most part on the guide.
Keep in mind that it is tough to accurately give a range for the kits, because depending how how SNK distributed it's games, i.e. there's much more puzzle games released in Europe than there is in the U.S., so for a game like Pop 'N Bounce, it is considered very rare here, but in Europe, it's not as rare, therefore you can buy this game much cheaper in Europe than you can here. Notice all the European sellers posting no U.S. Ebay. It's because they know we can't find certain titles in the U.S. and they'll make more from us then from someone in their country. Vice-versa.