Consumer group Which? has accused Apple, Google and other tablet manufacturers of "outrageous" mark-ups on models with more memory.
Its investigation found that the average market price for adding an extra 16GB of capacity to a device was just £5.95.
But the it found that Apple charges an extra £80 for a 32GB iPad Air compared with the 16GB model, and other manufacturers also charge way over the odds.
Which? advises customers thinking of buying a tablet to opt for one with an SD or microSD card slot instead of internal memory.
Other offenders include Google, which charges an extra £70 to add 16GB of memory in its Nexus 10 range, and Amazon which charges £40 for the same storage increase in its Kindle Fire HDX 8.9.
The difference in cost between buying a 32GB device or a 16GB one are as follows:
- Apple (iPad Air) - +£80
- Google (Nexus 10) - +£70
- Amazon (Kindle Fire HDX 8.9) - +£40
Which? editor Richard Headland, said he thinks potential customers "will be shocked to discover what a raw deal they're getting on built-in memory".
Researchers also found that many devices do not give customers the capacity that is advertised, since this tends not to include the memory taken up by pre-installed apps.
For example, a 16GB Samsung Galaxy Tab 3 comes with only 10GB for the owner's songs, apps and photos.
Manufacturers use a range of different suppliers for their memory storage, but the price of such components is tracked by market watchers such as DRAMeXchange.
Which? says that the average price for 16GB of memory, as tracked between August and October of this year, was just £5.95.