JYDGE

Last weapon mod opens up at medal #94, which means that you’re probably at Act IV already and you have enough money to buy all the non-purchased ones in one batch.

The total credit requirement is 30,000 but you probably have bought plenty of weapon mods by this point of the game so you don’t need the full 30k. A good map to grind for some money is Shift From Hell on Hardcore. With a well equipped Jydge you can easily collect 4-5k in a 2 minute long bloodbath.
(a maxed out shotgun with long range and killing heals + good shields and 150% health which you can heal back were my choice).

As a reminder, here are the weapon mods of the game:
600 Reload plasma
650 Citizen safe ammo
700 Killing heals
750 Rapid fire
800 Sandman
850 Piercing lead
900 Extra special
950 Power gavel
1 000 Melee deflect
1 050 Long range
1 100 Bull shot
1 150 Dynamite gavel
1 200 Murderous lead
1 250 Fast reload
1 300 Pain gavel
1 350 Overcharge
1 400 Mighty burst
1 450 Extended magazine
1 500 Melee knockback
1 550 Tough reloader
1 600 Lovestruck
1 650 Fast melee
1 700 Explosive bullets
1 750 Blood specials
1 800 Crysis protocol
30 000 TOTAL