In a well-functioning market, the price should be driven by
competition towards a level a bit above the production/R&D costs
The main issue is that R&D is largely a fixed cost. It takes the same amount of engineers to write the Bing engine or the Dropbox I/O layer, no matter how many users are using it.
How do you decide the strategy for recovering a fixed cost of millions of dollars from the price of the individual product? Price all DropBox accounts at $10 and you may not have enough margin dollars per account to cover your R&D. Price all DropBox accounts at $20 and you may not have enough volume to... cover your R&D.
Assuming the incremental storage for 50 GB costs $2, it's ok to recover $8 and $16 respectively for fixed costs. Just because they are fixed doesn't mean that the user shouldn't have to pay for them, or that it should pay in the same amount. You could argue even that the R&D should be paid in a 2:1 ratio as well by those benefiting from 100 GB versus those benefiting from 50 GB. It's the risk of the entrepreneur to decide where to set the bar so that, at the end of the day, the margin multiplied by the volume recovers the fixed costs.
Competition driving the price towards a level a bit above the R&D (fixed) costs doesn't work at all for products where the marginal cost is trivial but the R&D expenditures are huge. Why Ask, with 1-5% market share, cannot compete with Google Search, which has more than 60% in the US market? Crawling the web, building the index and removing spam costs more or less the same in terms of R&D if you aim for an approximative identical standard of search quality in the search engine. But having a market 10 times greater or 2 times greater (in Google versus Bing) means that one allows himself 10 times or 2 times the total fixed costs at equal levels on a per user basis. No wonder Bing losses millions while Google uses the ad revenue to subsidize its other operational branches.