No it doesn't highlight a lack of ambition. If you have someone like Kilman who is potentially good enough to be an England international, why on earth would you spend 30 mil on someone else to stunt his growth ?
We spent 35 mil on Fabio - if that doesnt spell ambition, what does ? And we were probably wrong too.
Money doesn't always buy happiness.
You're relying on your players growing at a rate with no decline (which comes naturally with age), and that other players do not develop at a similar / faster rate.
If you stand still, you fall behind by virtue of other teams spending money and improving their squad.
Have our centre backs improved over time or have they declined?
The Fabio comparison doesn't hold water, we're in an era where championship clubs can command 40M+ for their players, if we got relegated and Fabio had a breakout season, then they would see a return on the investment. It's relatively safe.
In terms of expenditure, teams that spent less than 10M (excluding Chelsea due to embargo and the fact that they literally signed Thiago Silva):
Cardiff
Sheffield United
Newcastle
Norwich
Huddersfield
WBA
Watford
Wolves