Making such an investment is fine, if there is a reasonable chance of it paying off.
Can Adsense provide that chance? I tend to think it can, depending on if the site topic can support that kind of return.
Basically, you need to plan today for what might happen next year, so to speak.
If Adsense can provide that kind of income, there's a distinct possibility that other forms of advertizing can also.
Using Adsense to establish the advertizing viability of a site is one way to look at it. It can give you a baseline number to use when talking to other potential advertizers in case Adsense takes a hike for whatever reason.
That said, perhaps a more important consideration is what kind of traffic the site has, that is, where does the traffic come from.
If it's soley from the free serps, I'd be very careful because the traffic can vanish as fast as Adsense income.
Even if traffic is generally from PPC ad campaigns, I'd still be leary of sinking too much money into content. As we've seen a number of times PPC can suddenly get very expensive. If a site can't support an unexpected jump in PPC costs, it could be in trouble quick.
If a site can generate enough traffic from quality links or offline marketing then these other factors become a bit less critical.
Of course $10,000.00 can buy a wide range in the quality and quantity of content.
I'd guess that content that can support a $10+ CPM will cost a lot more than content that can only support $1.00 CPM.
In the end, if $5,000.00 can buy a respectable amount of high quality content, I'd be likely to spend the other $5,000.00 on offline marketing.
But the site subject could play a big part in that kind of decision. I think what I outlined would work for me, others milage may vary.