Everyone seems to say Sienna is super trendy, am I the only one who's outdated here? =) Their color is neutral and the style doesn't scream OMG. Maybe, it's just me. I don't like the new Sienna FLoral Shoulder Bag though. Strictly personal preference, Sienna/Scarlett is the only style from Kooba that I like and own (tried it on & the proportion's right for me - big concern).
I think Kooba's price has gone up because it has obviously gained a big following (how big, i don't know) -- there are people who are willing to pay for it. Ripped off or not, I don't know. Many people said that the brand is established since Sienna Miller (the bag was named after her) wore it.
Price and qualities don't always go hand in hand. Chloe's been raising their price as well, but the leather of their bags don't seem to get any better. Their latest badge of Tan bags are incomparable to those from previous seasons. The same thing can be applied to Ballenciaga bags, I believe most fans agree that the recent leathers aren't as nice.
I usually look at the style first, then quality. If the bag is nice but quality's not too good, I don't buy it and vice versa. Price matters too. Brand doesn't dictate my choice though, I wouldn't root for a single brand and buy bags made by them only.