Before I say anything more, please check out this page:
Quarterback Contracts and Salaries | Over The Cap
Unless another QB signs a contract into the top 5-10, we can probably safely assume that transition tag will be ~$20.5 and the franchise tag will be ~$21.5.
I'd argue that today it's pretty difficult to get anything other than a journeyman QB for less than $10M/year... and the salary cap is supposed to be much higher this offseason. So, in future dollars, Kirk Cousins will probably do pretty well on the open market. If you want a 1-year rental on a player as a gap solution, you'll also likely pay a premium over someone who wants to invest in Cousins to see if he is the QB of that team's future.
I've said a lot about this topic, so I'll try to just sum it up this way... It's unlikely that Cousins accepts less than $20M guaranteed money in the 1st year to sign before hitting the open market, because that's where his leverage is. Maybe a 2 year, $24M deal with $20M guaranteed makes sense for Cousins & maybe team too. You basically give him the Nick Foles deal, but he gets more guaranteed in signing bonus and it would still be a short-term deal. You can add a year and give him more money over time, but you have to give him the $20M in the 1st year (because that's what he'd get if you transition him). You cut him in 2018 (post-June), you create $10M in dead cap in 2019...
If he's not worth this, you let him hit the open market and then you either try to sign him back for less or go with one of the other available QBs... It's just numbers... it's business. That's how it works out.
Cousins isn't going to accept less money for an opportunity when he can have money in hand and get an opportunity somewhere else. We've proven we aren't a stable situation for any QB, so it's likely we'll just have a journeyman next year. Just one man's opinion...
If Cousins accepts less, there is something keeping him here... Family or something like that.