First i will link this for reference.
Cornerback Contracts and Salaries | Over The Cap
Here is what I think Butler should be paid. The elite of the elite CBs make about 15-14M a year these days. While that might be desperate teams willing to overpay a little that is what the absolute top of the market is today and we can expect salaries to continue to go up over time as the cap goes up. That is what guys like Revis, Peterson, Norman and Sherman make.
So the question is this "is Butler that good or likely to be that good". Well he generally is not that good right now and i don't think he will ever be top 5 cause he does have certain physical limitations. So the absolute top of the market is not for Butler now or in the future though i think we can expect him to improve just a bit more.
So what was Butler's play worth last year? He made a probowl and i would argue rightly so. He certainly belongs on the short list of considered names worth debating about at the least. I would say most people would say he was top 15 last year overall and arguably top 10 (I put him in the top 10 considering the competition he faced)
Lets see what the 10th highest paid CB made last year... Brandon Carr is #10 on that list and made 10.2M that year.
Looking at players who are established which i think are right there with Butler skill wise though their games might be different (this is now my own personal opinion) I put him with guys like Talib, Jenkins, Flowers and Harris. They make 9.5, 12,5, 9.1 & 8.5M respectively. So lets call it average close to 9.5M wichi is in line with that the #10 best paid CB made last year which fits.
Now lets assume any deal we make with Butler will factor in the cap going up and the longer the deal is the more we can likely save on a per year basis. Generally thought Butller is worth 10M a year now likely to be worth 12-13M in 5 years due to increased skill and raising of the cap.
So to pay him a fair deal for what he is worth would look like 9.5, 10, 11, 12, 12.5 or averaging out to 5 years 5 years 55M (11 per year). However we do have leverage to hold him to a little less but i think the more the Pats play ball with Butler the more likely he is to play ball with the Pats. Pay him a little more now to pay less later and give him more guaranteed to make the overall salary less.
right now he lets say he has a 3 yr deal where the 3rd year is 15m (franchise tag) the Pats will not pay 15 M so we get him cheap for 2 years if we play hard ball then he probably goes to seek top market since we didn't pay him and probably has hard feelings over it.