Is the stash outpost worth it? I just reached silver league

Since yesterday morning, I've had 31 pickups total. I've been quite diligent about pickups since the event began, so we can probably consider that a top-line level of pickups. Averaging that out, that means I'm likely looking at ~20 pickups per day. If you get the same amount, that would mean an extra 20 event currency per day for you, over the remaining 20 days of the event, or 400 event currency (EC).

The stash outpost costs $8. That means you're getting basically 50 EC per dollar spent.

If you choose to purchase event currency with diamonds, I'll save you the math, but it's about half as efficient on a per-dollar basis as buying the stash outpost (assuming you're buying the 'reasonable-sized' packages like the $20 -- if you're buying the $100 diamond packs, it gets closer in cost efficiency).

So, I'd say it's "worth it" in the context that it's likely the most cost-effective method of acquiring additional event currency -- if you are planning on spending money on this event, this may be the best place to spend it. But whether it's "worth it" to

*you* is an entirely separate question. I personally won't buy it because the extra rewards for Silver/Gold aren't worth it to me (and honestly, the stash outpost won't even guarantee I get Silver anyway).