To be pedantic - they didn't slash their price. It's still the same. They are now just offering additional space.
The economics of this is interesting. Drop-box does not store multiple copy's of identical files for each user. So, their cost per GB drops as they add users. Which also means if they start to lose users they may need to raise prices, which could quickly cause a death spiral.
Also Not storing multiple copies of each file is really bad product-wise as there were some incidents where users lost their data stored on Dropbox with no other backups (relying heavily on cloud storage).