Currently one must pay 750 energy as well as acquiring the monthly lockbox itself to open it. Unfortunately, a majority of these items -- accessories -- are worth far less value compared to the box opening.

Therefore, I propose some suggestions to make the box opening easier:
-> A cheaper key: the Copper Key. This key costs 200 energy to purchase and may only be used to open Monthly Lockboxes. Silver Keys remain as an option to open Slime Lockboxes and perhaps more valuable lockboxes that could be introduced in the future.
-> Bring back the "Open 2 Get 1 Back" mechanic: a few months ago players were guaranteed a lockbox from the prize wheel after unlocking two lockboxes. Not a perfect solution but one that directly rewarded box-opening.

While hypothetically the value of items within lockboxes should increase with how difficult it is to open them currently, there are far too many of these accessories in circulation for them to increase in value significantly. It may take over a decade for these accessories (vees, headbands etc.) to recover their true value.