As an FYI, here are contract numbers for NFL left tackles (in addition to the article cited in the original post).
Note: you can use the drop down menus to change from 2017 to 2018, 2019, full length of contract, etc.
Left Tackle Contracts and Salaries | Over The Cap
Left Tackle Active Player Contracts | Spotrac
NFL Salary Rankings and Cap Hits - LT | Spotrac
Since his original rookie contract expired, Nate Solder's cap number has ranked 2nd among left tackles (in 2016, the first year of his extension) and now 5th in 2017.
Right now the 5th cap number for a LT in 2018 would be $13,950,000; 10th would be $11,400,000. For 2019 we are looking at $14,200,000 (5th) and $11,700,000 (10th). That should give us a benchmark of what it would cost to extend Solder - assuming of course that he even wants to continue playing beyond this season.
One important aspect to keep in mind: we already know that the salary cap is going to increase significantly over the next few years based on television contracts that have already been signed. We are looking at base salary caps along the lines of:
2015 -- $143,280,000
2016 -- $155,270,000
2017 -- $167,000,000
2018 -- $178,000,000
2019 -- $190,000,000
That is a significant consideration. What may appear to be an outrageous contract that does not make fiscal sense today, could actually represent the same (or even lower) portion of a team's cap a year or two from now.