We will be launching a new guild level in the next few days. The Dark Assassin. The pricing for the new level will be 900,000,000, to bring the maximum spy cost more in line with the maximum tier 3, level 3 attack build cost.
There will also be a level for spy defensive tower. Note that spies won't get plunder bonuses ever. Spy buildings cost much less than attack buildings, and at the same cost, they generate comparable income without depending on allies (which might get stolen from you and varies far more). Spy builds also have numerous other advantages to attack builds (e.g. not being exposed on a successful action, various not-attackable cases, etc.) As such, the income made at a maxed out level 4 will still be less than a maxed attack build with ally bonus. Note however that a fully upgraded spy build (at less than 1 bil) still costs less than a fully upgraded attack build (1.3 bil). Choosing a spy build means tradeoffs, so make your decision wisely.
The new level is launched, simply restart the game. There may be a display issue after you buy the first building, but it should fix itself if you restart. We're working on tracking this down.
For those wondering why their plunder might have dropped, if you're hitting the same person you were before, your plunder will definitely be less. Level 4 increases your strength, probably bumping you into a higher level. Spy builds are now on the same strength levels as attack builds, so you need to hit stronger players.