DoD upgrades are not always cheaper in aggregate; it depends on what fare class you're on. KLM has a specific table they use that decreases the copay amount about 50 euros for every single fare class higher your ticket is in Y. This is used for all DoD upgrades on KLM and DL ex-AMS.

The OLCI and kiosks sometimes offer you cash only offers. My example on it not always being cheaper: kiosk offers a 329 euro upgrade, but the lounge using their table would want 400 euros plus 25K miles (the miles are always the same; the euros amount is what changes, as low as 100 euros and as much as 450 euros, meaning my ticket was a pretty low fare class).

KLM offered me an onboard upgrade (after I asked the purser) for 1400 euros from Y to J (AMS-ATL). Declined.

AZ offered me an onboard upgrade (again, after I asked) for 250 euros from Y to W, and 1000 euros from Y to J (JFK-FCO). Also declined.