I wouldn't be shocked if Len got a deal just a little higher than what Williams received, any more than that would be a mistake. The main reasons for signing him to a contract at all would be to be to use him in a trade at a later date, as an asset or making salaries work, and also so we can let Chandler go. If he signed for around $18-20 million over 3 years, we should be able to move him easily if needed. He may even develop but I won't hold my breath waiting for that. Having him signed would allow us to trade Chandler though, which needs to be done this offseason if we hope to get anything in return for him, even if it's just a late 1st rounder down the line.
I don't think Len has a long term future here but he should factor into our plans longer than Chandler. Having Len locked in would allow us to move on from Chandler and let us to look for a replacement that fits the new NBA better. Have Len play a limited role on the team for 20-25 minutes similar to how Golden State uses Zaza, as a rim protector and rebounder in the first half and if the game is slowed down in the 4th.