When upgrading, you don't only need the unlock from the tech centre, you will also need to pay for the new units.

Generally I would take the mining upgrades to a similar cost before trying to invest in the upgraded bots. Also, generally make sure to invest into a little bit of the speed upgrades (at least level 1) before trying to unlock the new type, to ensure there's enough income coming in.

Otherwise, once you can afford it, each one is at least 10x better than their previous counterpart. Note that if you stop using something you get back the full price.

• DB-1: $25
  
• DB-2: $25k ($25,000)
  
• DB-3: $250m ($250,000,000)
  
• DB-4: $1.50T ($1,500,000,000,000)
  
• DB-5: $10.0P ($10,000,000,000,000)

• RM-1: $100
  
• RM-2: $40M
  
• RM-3: $4.00P

• MM-1: $200
  
• MM-2: $120m

Tech points - you can basically always autobuy these. Research points and tech points are shared between every grid.

Generally mine capacity helps a lot up to level 4, as fitting more bots into one slot reduces the power overhead required to run the mine, letting you buy more bots overall for the same power grid. A tier.

Bot power efficiency: You won't need these when the bots are new and money is the limit, but becomes S tier once you run out of power to slot new ones in. Each halving upgrade basically allowing you to double the bots you have. At the very least, A tier.

Bot speed: pretty good, but only equivalent to 1-2 levels of mining buffs. Bots waste a lot of time walking. B tier.

Buff slots - Really good at the start. Allows you to additively combine buffs, adding them together. I usually like to keep the top buff from fuel always going and then split the rest, switching between tech or mana depending on what I need next. Loses out in the end because you should always install the top level buffs first, so incrementally you only get to install the middling low level mana buffs. S at the start, C at the end - B tier overall.

Extra floors - eh, pretty low priority unless you are literally maxing out each floor at 16 mines and can't fit any bots. Spreading out the bots can grant something like a 5% bonus but this is pretty minor. C tier.

New unlocks - generally these are exceptionally good, but only if you can afford to buy them (see the first section). A tier.