The cost is not that significant. Heck, the whole BTC market cap is not that significant.
If there were 6 GB block size bloat per hour?
A financial attack could do this independently.
Miners could do this free-ish.
Small miners would fail, as would all hobby miners.
Full nodes would become centralized, increased 51% risks, etc.
These are just the obvious. No more decentralisation for Bitcoin.
From the wiki:
Note that a typical transaction is 500 bytes, so the typical transaction fee for low-priority transactions is 0.1 mBTC (0.0001 BTC), regardless of the number of bitcoins sent.
To spam the 1 MB blocksize takes roughly .2 BTC per block, or 1.2 BTC per hour. That's only $500 per hour.
To spam a 1 GB blocksize takes roughly 200 BTC per block, or 1200 BTC per hour. That's $500,000 per hour!
A 1 GB blocksize is far more costly to attack. We could increase the blocksize to 1 GB now and nothing would happen because there aren't that many transactions to fill such blocks.