Tanev is currently in the last year of his deal for the Flames where he makes $4.5 million. If a team were to ask the Flames to retain on Tanev, it would cost considerably more as they would be asking the Flames to keep $2.25 million against the cap and over a million in real dollars.
Tanev with 50% retention is likely costing much more than a first and is likely closer to two first-round picks than just one. We will wait and see how much he goes for when he is moved but if one thing is for certain, it will not be cheap.