Not just that, but it's reasonably common to see the initial small-looking campaign suddenly jump to airline-bleeding proportions, with you having no way to cancel it, because you no longer have the cash to actually pay the cancellation fee.
Happened to me in JA8...and my airline was not so new...once the cash crunch happened not being able to cancel contributed to a death spiral.
I found the campaign structure convoluted, and agree that it obscures the relationship between amount spent, CI growth (or RI), number of routes/aircraft, etc.. For noobs, unless there are published tables that report these, there is no way of knowing unless they/we experience them directly themselves. The Manual is murky, at best, about this.
That said, if anything, it would be nice to have a grace period, even if it were only for 24-48 RL hours to be able to switch campaigns or to cancel at a reduced penalty fee. If longer than 24-48 hours, tie it to the number of hours of game time (age of airline/s) that the player has played on AWS might be reasonable to give noobs some flexibility.
I don't favor a cap, but instead would prefer to be able to set the nominal amount directly and three values calculated from that and displayed, along with change from previous week: 1) Spend per current seat flown amount, 2) Per current CI amount, and 3) Percentage of current operating revenue. Since these three will be different for each airline type (mix of LH vs SH), they should provide a general sense as to the affordability and effectiveness. Alternatively, the player sets the percentage of current operating revenue to allow the amount to grow as the airline grows in revenue (vs other factors that cause the spend to grow faster than revenue today in AWS), which might help lower the administrative aspect of this.
Along with that, better than a grace period, a player should not have to commit to a campaign but be able to dynamically adjust the amount spent on marketing. If there is a penalty/fee for each adjustment after it is saved, fine. This may be less IRL simulation, but perhaps better for gameplay, IMHO.
This mechanism (and indicators) with the grace period/dynamic settings will go a long way in helping noobs understand the relationships and in seeing the impact of their decisions while leaving the door open for them to change their minds.
I would also like to see the same mechanism applied to both geographic areas (what I assume might have been the original intent with the current marketing structure) and to specific routes. But that is beyond the scope of this request.