I think there are some really nice kate spade bags out there that are underrated. Since they go on sale so frequently, you can really get some deals.
I also think the Sak bags are underrated, they are so functional and cheap. I know the crochet look isn't for everyone, but the line has some fabulous and diverse options now that aren't just crochet. Even their leather bags are good quality for the price. Plus you can almost always find the Sak bags on sale for $30 or less, I think they're a good buy for a casual bag.