1. Well, it would be inadequate to sort by full price when some games are very often on sale at 50% off, even more. (Indie games, notably.)
There isn't really a metric to estimate a game's value, so I tend to use g2a.com prices (even though I'd never buy over there), if only to compare the prices of two games with different ratios, to determine if some common game has more market value than a rarer game, and if it's worth trading for it. At that point, suffice to say the retail price point doesn't really make sense to me. If you can give me some examples, I could look into it, but... :-/
I could, however, possibly allow sorting by 'lowest price recorded'. That'd make more sense, to me. Even though it always has limitations -- if a game went 80% off on a special weekend years ago and hasn't moved since, it really shouldn't be considered as a viable metric.

2. Hmm, that'd be a nightmare for long lists... Plus, one of Lestrade's things is that the entire system gets you as much information as possible in as limited a space as possible. It's a hard balance to maintain. Adding banners here would force me to put both lists on top of each other, as opposed to in front of each other, making balanced choices less natural to me. I could possibly add a banner when hovering over the title, _but_ it'd only work on desktop computers, not on smartphones.