SRAM/DRAM Price Difference
So one could guess that for the same amount of information (say, 1GB), SRAM would be around 6 times more expensive than DRAM (if capacitors were free).

Actually, making the capacitor is the hard part.

In the real world, however, this difference seems to be much larger than a factor of 6. What's the reason for that?

SRAM itself isn't that expensive when it's made at high volumes. Go buy a few hundred dollar GPU and you're getting a lot of SRAM, in some cases more than 100MB worth ... as well as thousands of graphics cores and gigabytes of GDDR.  Factor in the fraction of the die that is actually SRAM (not that much) and the fraction of the cost that is the die and you aren't paying that much per MB of SRAM.
But if you're looking at discrete 512 Mbit SRAMs (rather than a similar amount of SRAM integrated into a GPU or similar mass market product), that's an extreme niche part made in tiny volumes and priced accordingly.