At $3.9M, an offer sheet only costs a 2nd. I gladly pay a 2nd and offer as many years as he wants at $3.9. So that is setting the stage for his compensation from Washington. If they won't budge off of 2 x $3, or 2 x $3.5, I guarantee he could get $3.9M for however long he wants somewhere.
In terms of trading for his rights, any team that thinks he may be their long-term starter is going to be willing to pay more in assets and salary than Washington who already has Holtby at $6M and one of the Top 3 goalie prospects in the league. As goalies don't usually go for much, I don't think it would be that expensive to trade for him. He does not have to sign the QO at $1.5M. The power of the RFA has dwindled as players have threatened to hold out, basically forcing a trade. If you don't trade them, you lose them for nothing. By not signing, he is giving up the $1.5M, but there is no risk his value would dwindle due to injury or poor play.
As an example, Schneider had 100 career games, no season over 30 games and a $4M price tag when the Devils gave up the #9 pick for him. Its not the exact same comparison because in Schneider's case, Vancouver signed him, gave him another year as back-up and then traded him. But for the Devils, they were taking on a $4M 27 year-old career back-up as their starter and paying a heavy price to do so. The cap has gone up now by over $10M since then, so as a percentage of the cap, it would be equivalent to paying Grubauer $4.85 with a $78M cap.
If you are Blues GM and could move Allen (or NYI/Buffalo GM and don't have to move anyone), would you pay $4M for Raanta? Is he any less of another team's back-up? He only had 45 games for Arizona. If he signs elsewhere, as the Blues GM would you rather pay Allen and pray he finds consistency or trade for a goalie that you think would work with your system and had success elsewhere? Obviously that comes down to price, but goalies rarely go for what Schneider brought. If we could trade Allen for Buffalo's 2nd and get Grubauer for Sanford, a B-prospect and our 2nd, would you? I do that deal easily, but I am more down on Allen that you.