Tl;dr Aquisitions do damage to the character’s Cashflow, Earnings act like Armour and Shortfalls are Harms.
Just like physical damage this means that damage to Cashflow is mostly not an issue unless it causes a Shortfall before you can heal it.
In practical terms it means that you:
simply don’t tract incidental purchases with a cost below the characters Earnings.
everyday purchases will cause a little bit of damage to Cashflow, but these only matter in aggregate
large purchases that cause Shortfalls are where players start to feel the pinch.
Equally, this means that recovering Cashflow isn’t a big deal (in comparison most characters will recover all Vigour or Morale damage with a good night’s rest). Rather, what matters is recovering Shortfalls.
This is what the rules say about that (page 331):
by spending an Asset and making a Simple (D0) Lifestyle test. For each Momentum spent from the test, the character may restore two Cashflow
or remove one Shortfall. The difficulty of this test increases by one step for each Shortfall the character is currently suffering from. Additional Assets can be spent to add bonus d20s to this check.
So what this means for an average character (Lifestyle TN ~10) recovering from multiple Shortfalls will be an expensive proposition: at least 1 Asset and realistically at least an additional Asset for each additional Shortfall they have (for a D2 test you want to be rolling at least 3 dice).
It’s worth noting that the most Shortfalls buying any one item can cause is 2 (1 for 5+ Cashflow and 1 for reducing Cashflow to 0). The issue is “provided that they can find one for sale”: the difficulty in Aquisition is often not paying for things, but rather finding them in the first place when you have a schedule. Remember that all the usual ways of modifying Skill tests are available to you as a GM for Aquisition tests (so if the player is trying to purchase something in an environment where they don’t fit, then that’ll increase the Difficulty of the test).
Take TO, for instance, it’s a D4 Aquisition test to find but will only ever cause 2 Shortfalls. So the issue is less purchasing it, it’s finding it available in the first place.
Ultimately this means that Cashflow refilling trivially isn’t really an issue. However, most of the time I’ve seen the Cashflow refresh every [narratively useful period of time]. I’m 6 sessions into a campaign and we’re still on day 1, so I haven’t let my players refresh their Cashflow from their pre-game spending yet. I expect we’ll go through the entire mission before I let them. Just like opportunities to sleep, Cashflow-refreshing is something that’s within your purview to control: if a player really needs to restore their Cashflow on a faster schedule, then there are other options for that.
Essentially, competent characters in Infinity can get a hold of anything they want given enough time and effort. The Aquisition rules function as a narrative tool to temporarily gate access to objects to serve the story.
Re: the Geist upgrade itself. Depending on how extensive an upgrade he wants, its either very expensive and relatively time consuming to get and potentially causes glitches (and probably causes a bit of a personality change) when intergrating with the geist or, its an extremely simple new app that the geist can download and install itself for no change.
Mechanically, the game distinguishes from new Attributes and Traits (these both significantly increase the cost and complexity of the upgrade) and skills/talents (which are relatively cheap and easy to get).
A major upgrade to a Geist’s attributes (or buying a pre-packaged Geist) is probably going to result in Shortfalls. It’s something a character will want to do once.
OTOH, something I like doing as a player is buying a 1 EXP / 1 FOC + first level talent upgrade that’s relevant to the module I’m doing. This is quite cheap (it’s not likely to cause a Shortfall) and usually means that any Assists the geist performs will not be Untrained and will have a reroll (most non-combat skills have a reroll talent as the first talent).
IE. One of my characters is playing in a game where we’re doing a Cube recovery mission in CA held Paradiso. I bought a Medicine 1 EXP / 1 FOC / Physician upgrade to help with any First Aid.
Unless the player has a combination of high earning and lifestyle expertise it will quickly become difficult. Remember every 5 subtracted to cash-flow generate a shortfall, and the upgrade will be progressively more difficult. It’s rather easy to have a decent well rounded Geist, having a very specialized one, or even better a Geist which is good at everything will cost a lot. Cost can be reduced if purchased in package, but still cost a lot for a one time purchase. I’m not a 100% positive because I haven’t faced this situation but I doubt it should prove a major issue. If it does just refresh their cash-flow at a slower rate, like some regular milestones.
I run upgraded Geists in pretty much every game I play.
You can get a AWA 9 / INT 9 Geist with a couple of skills for ~1 Shortfall if you are careful with how you build it. Once you have that base then buying upgrade patches applicable to the task at hand to get 1/1 + Reroll Talent can give you a TN10 + Reroll Jackgeist that covers off on most skills a LAI can easily make use of.
OTOH thats only 75% chance of +1 Success on tests the GM allows you to use the Assist for. It’s certainly useful, but it’s not even close to broken.