16m per is NOT solid starter money, AB @8 is solid starter money. 16m is star money b/c it's max for him. Sully may not be a defender (lazy) but some of you would lose it if we only offered him 10, and Sully is a big with a lot of potential left too! Many people make less than 16m and are better than he is!
There are a few things you need to consider here.
Firstly, AB was signed to $8M when the cap was up around $58M - this would put his deal at about 14% of the cap a the time of signing. That's roughly half of a max contract and, as you said, about right for a solid starter like AB.
If I understand correctly Barnes is still under contract for this upcoming season, so if he signed this extension the new contract would start in 2016/17, when the salary cap is expected to rise to about $90M. In that case 14% of the the cap (equivalent to AB's deal) would be around $13M, so a $16M deal for a player with Barne's upside is really not excessive at all - it's about equivalent to $10M / year when Bradley signed his deal.
Then once you consider that the cap is expected to rise even more in the years after this (to over $100M) Barnes' $16M deal suddenly becomes roughly on par with what AB is making now.
Now Barnes (only 23 years old) has excellent size for for the SF position (at 6'8"), he's an outstanding athlete, he's been extremely healthy (has missed only 3 games out of 244 so far in three seasons), he's a quality defender, he can get to the basket and he can shoot (career 37% 3PT).
It's also worth noting that while he didn't score at a huge rate this year (12.8 Points Per 36) he had very efficient scoring numbers for a perimeter player (48% FG, 40% 3PT). This tells me that he was playing very comfortably within himself, and that he likely could have (and would have) scored at a much higher rate if he wasn't playing behind Curry and Thompson in the offense.
It sounds a lot of money because I think we all still have our minds stuck on where the cap has been the last 4 or 5 years, and it's hard to make that mental adjustment. We are used to thinking of $16M as max contract money, and it'll probably take time for us to adjust to the idea that under the new cap $16M is really just 'solid starter' money.
This is why I think the Bucks have a ridiculous bargain with Monroe.
I see his a floor of Trevor Ariza and a ceiling of Paul George - in either situation a $16M deal would look pretty good by the time that deal stars.