Damn glitchy network, I posted this early, and it triple posted, so I deleted two, and the original one disappeared leaving none... so here it is again;
Stamkos' Contract was signed when the cap was expected to be 73 mil, while Duchene will be extended with a higher cap. Next year, the Cap I believe is estimated at 80 mil, by the time the contract kicks in, it's probably 85. You could view Stamkos' contract as a ~9.9 mil contract when adjusting for the rising cap, so yeah, somewhere between Kane at 7 mil, and an adjusted Stamkos at 9.9 is right. Duchene doesn't have near the history Stamkos had at the time of the signing, so he's not likely to approach 9.9, that's for sure. It's worth noting Stamkos' contract may have been driven down by uncertainty around his injury leading into the contract, also Tbay has a favourable tax situation.
Ryan Johansen signed an 8 year deal at 25 years old for 8 mil per a year ago. That deal had one year of RFA bought in it I think, and would only really cover prime years it's not a perfect comparison, but he's far more comparable in terms of accomplishments to Duchene than Stamkos is. Like Tampa, Nashville has a favourable taxation situation which may play into contract negotiations.
My guess is Duchene comes in around 8-8.5, but maybe we get lucky and sign him btween 7.5 and 8.