I don’t see any pinned messages for what must be a pretty frequently asked question so I’ll ask here:
To take Batman Year One #404 (8.5/$11) and 405 (8.5/$24) for example, why has Covrprice estimated a lower value to the first part than to the second part?
It’s wholly controlled by sales data. I recommend going over to CovrPrice and looking at the actual individual sales. I will say, it looks like people buy these batman books from MyComicShop at a premium so when those sales come in they often bump up the FMV reported. The recent 405 sales in the VF are from MCS and they are showing sold in the $29 range … so that impacts things. But as new sales come in from across other marketplaces, the price may drop.