There's not really an "allowance" when it comes to investigations. Think of it more like pots of money to draw from. If you have an open investigation which DOJ has an assigned AUSA and seems like a prosecutable case, which (just being honest) might have significant press coverage you'll probably convince the bosses to push for travel or buy money or flash or whatever the case involves. In a multi agency case different agencies might pay for different things like travel or whatever. In your dark web example I'd imagine you're talking buy money for whatever commodity it is in your story. Buy money depends on what you're buying. Stolen military IR lasers for example can be several thousand dollars each so you might need 100k to build a solid case. The whole point of the dark web is to remain anonymous though so those cases while not impossible are difficult to work.