Use the 400 merits to buy CONVERTORS sell those on the black market/auction house, or use them to convert cheap enhancements to the ones you want.
I think this is correct (math):
10 merits gets 30 convertors.
100 merits gets 300 convertors.
300 convertors are worth 300 x 70,000= 21,000,000
More efficient could be to buy cheap enhancements and use the convertors to get what you want.
Example: Need a Gaussian's Synchronized Fire-Control?
Buy a Adjusted Targeting
Use a convertor to change the Adjusted Targeting to another Category: ToHit (spot to do this shown in below image as Category: Healing)
Bingo Gaussian's Synchronized Fire-Control (cost 2 convertors)
Reference:
https://hcwiki.cityofheroes.dev/wiki/Enhancement_Sets
Convert menu on enhancement tray.