Cobb versus Hellickson is an interesting comparison. Cobb has been a bit better pitcher his entire career than Hellickson up to that point. Hellickson was more durable. Most teams bet on upside though so the better pitcher (Cobb) would likely get the larger contract all other things being equal. If one believes completely in FIP Cobb has a career mark of 3.68 versus 4.51 for Hellickson (and it was still probably about 4.40 last year). Cobb gives up fewer homers and has a higher K rate.
The QO (which will be $18 million this year) is important in why Hellickson did not get a great offer. First, compensation was greater last year under the QO so teams were more reluctant to make offers. The old QO was probably worth about $20 million in contract value. It was reasonable to think that Hellickson would have gotten lots of 3-4 year offers that were at least $15 million a year without the QO. Think of it this way, if Hellickson got a 4/70 offer from someone it was the equivalent of a 4/90 offer and that was above Hellickson's value.
The Rays will almost certainly not offer a QO here. Which frees Cobb to get offers north of $15 million a year. As I said, he is a bit better than Hellickson and should be valued accordingly. Obviously he'll be the 5th or 6th choice on the market so what he gets will be dictated in some way by what happens above him.
Cobb would not be my first choice. Obviously Otani but I think I would go for Darvish, Arrietta, and Tanaka all before Cobb. I am a little wary of Lynn, especially since he might get a QO. The Phillies should go for quality here and just pay what it takes to get that. I'd prefer one pitcher at the top of the market than 2 from the middle.
And BTW what happened to Hellickson this year has nothing to do with what offer Cobb should get. All pitchers have risk. They get paid more as free agents because you do not have to give up players to acquire them. There are almost as many long term deals that work out on $/WAR basis as ones that don't. It is also indisputable that teams can acquire WAR by spending money and the Phillies are in a unique position to be able to handle the cost.