Enforcing gold from only biter kills would be a good idea, since nuking entire forests can provide hundreds of thousands of gold within minutes. Which is what I ended up doing to test all the items to see how they work out.
Other than that, I find the mechanic quite interesting and I look forward to seeing its growth in the coming months.
Damn forgot to fix it..
But how much gold did you actually get from farming trees?
i'd say 1000-5000 would still be okay (on default price multiplier)
if you just want to test, you could also enter /c remote.call("rpg-items","add_gold",game.player.force,1000000)
I obtained 1.2 million gold within 10 minutes of nuking forests and biters that happened to be within them.
Although, I first tested the idea by gunning down trees, which would give a strong buff early game even despite pollution concerns as uranium ammo can be obtained in under the first hour if forests are dense enough. That alone would negate any biter attacks since uranium shreds nests and makes turrets unstoppable when buffed further.
I won't even get into the other powerful item buffs (or mass skill point book purchasing) you can apply within the first hour, since you likely get the picture.
It is, though with plates automatically created (Drill tech tree) you could stack several of those and hand craft all red/green science in mere seconds and jump right into blue science in no time at all.
That can be done with the starting money straight away, but balance has been restored and the forests are saved from genocide to prevent heavy abuse. Though part of me feels starting money should be reduced to prevent plate creating drill purchases which bypass the need to mine.
You've done a fine job creating lower tier items which combine for better and/or unique effects. The delicate matter at this point would be balancing costs based on progression and value.
Much like in a MOBA you typically get a simple boot for a minor speed boost, perhaps an attack or utility item (also very minor boosts), and maybe a few potions. These minor stat boosts are fitting for the start and provide a sampling of what the next tier will provide.
This seems to be what you intend to aim for and you've managed to achieve that to an extent, but some items are too strong for their cost such as the auto-plate drills, which as an intermediate non-starter item needs a price hike and/or their effects nerfed to better fit their cost. MOBAs are balanced regularly to ensure most options are viable without being the best irrefutable choice. Typically balancing with "Does this feel too strong at this point in the game?" as the main starting point goes a long way by itself.
I'd suggest trying different starts and seeing how each item path feels to find what's too good for its value. I've tested a few already and felt like the basic starting items were too expensive and the intermediate tiers too cheap for their immense value. That is why I suggested a starting gold reduction to prevent buying cheap intermediates outright which skew the balance, though starting tier items also need a reduction to open up some options so people can pick their preferred minor stat boosts.
I'm more than happy to provide balance advice time to time (It was my job once) but as the author it's ultimately your decision on the direction things go. It's primarily the early and mid game periods that need the most delicate work, though I'm sure in due time you'll find the right balance to give the players a smooth progression throughout the game while still being a badass late game with a couple final tier items to combine for great effect.