I wish there was a way in your account info > purchase history where you could
easily find out how much you've spent on apps alone.
I painstakingly went through all of my invoices since the opening of the App Store in 07/08. It's not an easy process but you can find out exactly what you've spent if you're determined and perhaps extremely OCD, like me.
Once in your purchase history, you've got to first navigate to the opening of the store July 08. From there you'll see a list of orders and columns for titles included in order and total price.
Here the tedious process begins. The problem is the field "titles in order" isn't wide enough to view everything on that particular invoice. If you see a fee in the "total price" column you've got to open that invoice to view the details and determine how much of the total was actually spent on apps.
Once you're finished with that invoice you click done and are taken back to the list of orders. Fortunately from there you can use the previous button to view the remaining invoices. Of course, you've got to keep a calculator handy and keep a running total until you make it through all of the invoices.
After doing all of this, I am quite surprised my number isn't much higher. I've currently got 195 Apps in my iTunes Applications folder. I've deleted others through time which were so ridiculous I knew I'd never use them again.
I've actually just spent $48.85 + tax (not OCD enough to figure out how much tax was for apps vs. media on each individual invoice) on all of those apps over a 14 month period and $18 of that was on MLB At Bat 2009 and Madden 10 alone.