I feel one should make 2 distinct differences:
1. Costs made to actually being operational in F1. I'll include costs for developing next year's car in this category.
2. Costs made to actually raise one's competitiveness. In season development basically.
Marussia last year was not able to operate on a pure operational level last year. The only update they made last year was taping over a slot between 2 wing elements. Lotus this year is in the very same situation, despite having tried to reduce costs massively. The same goes for Sauber.
Bhall, I do agree with you when say that in a spending competition teams have their own responsibility to not take it too far, that they don't raise too much the costs in category 2. However, teams should at all times be able to cover costs in category 1. You'll probably argue developing next year's car is a spending you can make as big as you want, and while that's true, this is also an essential part of F1. You can't drive around with last year's car year after year after year. Manor is doing that currently, but it's basically nothing more then going at track venues.
When several teams stop being able to cover costs in category 1, then you know there's something hugely wrong. I personally it's no longer a struggle to compete, it's a struggle to survive at all!