If you look at just the space you're using, then sure, Dropbox is expensive. But if you look at the extra services on top the of the fundamental sync/store service, perhaps it isn't. Determining the value is subjective - it depends on what you want from the service, how many of its features you actually use, etc.
If you've come to rely on the extra services and/or have an extensive network of friends, colleagues, etc. who you interact with via Dropbox, you'll probably view the service as being more valuable that just the amount of storage space offered.
In addition, one of the things that Dropbox banks on is people under-utilising the space on offer, which enhances the perception of it being good value, especially if you're a long-time user - the amount of space available to you has been increased over time (instead of the price being decreased). It also lessens their costs, as they only have to cater for the amount of space actually used, not what could be used.
Fundamentally, offering cheaper options would chew into their revenues, as many people who are happy to pay for more than what they use would jump to those options.
You might like to read the first and second responses here for more on this: https://www.quora.com/Why-did-Dropbox-move-to-a-single-Pro-p...
