IMPORTANT: If you meet the retail requirements already, then this achievement will pop upon acquiring the Forces of Nature DLC and exiting any match, be it Horde or vs. multiplayer.
Peter Knepley, Senior Multiplayer Programmer at EPIC Games, responded via Twitter to a question about the fortification levels:
Maximum Fortification levels (at retail):
Barriers- Level 8 ($170,000 total investment)
Decoys- Level 7 ($170,000 total investment)
Turrets- Level 8 ($360,000 total investment)
Sentries- Level 6 ($135,000 total investment)
Silverback- Level 2 ($200,000 total investment)
You level up your fortifications by spending money building/repairing/upgrading them. To determine how much the money you spend actually counts towards leveling up your fortifications, use the following formula:
Money towards fortification levels = (Money Spent) x (Difficulty Factor) x (1.0 + Mutator 1 + Mutator 2 + Mutator 3)
Difficulty Factors:
Casual = 60% = 0.60
Normal = 80% = 0.80
Hardcore = 100% = 1.00
Insane = 110% = 1.10
For a quick way to level up your fortifications, I suggest playing on Hardcore or Insane difficulty with Super Reload, No Ammo Pickups and Friendly Fire. Start at wave 39 on a map with a good funnel point for enemies- I prefer Trenches (behind sandbags in nest area) or Clocktower (behind the angled wall near a decoy close to the initial spawn). DO NOT FINISH THE WAVE- intentionally die to the final enemy, restart at current wave, spend your cash during the 30 second countdown, and repeat. Friendly Fire is to damage the Silverback so you can spend money on it.
Per Hotdogmcgee:
It's faster to level up via this method for everything except the silverback.
2 controllers, Insane, Friendly Fire + 2 other +10% mutators, doesn't matter which. I prefer Mercy, as you can have command posts with multiple decoys/sentries.
Before buying the Command Post, player 2 gives player 1 his money. Buy command post, sink the 2k into whatever needs leveled up. Smoke tag each other and restart from beginning. With a good rhythm, you can knock out a wave in 30-45 seconds, with re-loading, still easily under a minute per.
2000 * 1.1(insane)*1.25(mutators) = $2750 invested/minute, or 165k/hour.
Saves the trouble of messing around with a full wave of enemies, and you can do it by yourself so you don't have to split the winnings. This is also how I ground out my founder ribbons, since you get one every time too.
As always, if you feel this solution deserves a thumbs down, please explain why so I can make any necessary improvements. Thanks!