Look at EC2 pricing — instance pricing is pretty much a function of RAM size. I realize RAM isn't everything, but it is a large variable factor, with other cost factors either being constant or not growing proportionally fast.
I'd still argue that we should see bigger drops in pricing, especially for the large-memory instances.
They're really using RAM as a proxy for other real costs. The amount of RAM you allocate is a pretty good predictor for server/VPS utilization -- which means their actual power, bandwidth and hardware replacement costs as you wear out hardware faster. Virtually all unmanaged VPS and dedicated hosting companies make RAM the primary factor in determining the monthly service cost even though the price-per-gigabyte is more each month than it'd cost to buy the physical RAM outright. In other words, it's a mistake to think that the actual cost of RAM is the reason pricing is proportional to RAM size.
CPU is a better proxy for utilization than RAM, no?
CPU uses far more energy than a RAM, and I can write a low CPU web cache in 12GB RAM or a high CPU scientific simulation in L2 cache.
I'd still argue that we should see bigger drops in pricing, especially for the large-memory instances.