When people talk about the price of Bitcoin, they are referring to the current price at which Bitcoin is changing hands. Since Bitcoin is a purely speculative asset, this price is determined by how little sellers are willing to charge and how much buyers are willing to pay. Even then, the price of Bitcoin can vary across exchanges like Coinbase and Binance or currencies because of market inefficiencies
The other 2% of customer funds, held online, are covered in the event of a breach of Coinbase's online storage. Also, Coinbase holds all customer fiat currency in custodial bank accounts, on behalf of customers. So, if you have fiat currency in Coinbase, in a USD wallet, it is covered by FDIC insurance up to $250,000 (just like a "regular" bank). This protects customer assets (so long as they have been converted to fiat currency) even in the event of Coinbase becoming insolvent.
If you do have this much money tied up in Bitcoin, though, you may want a more secure space to store it. If this is the case, Coinbase offers a Coinbase vault, which has time-delayed withdrawals (giving you 48 hours to cancel a withdrawal) and the option of multiple approvers, increasing security by ensuring that all withdrawals are approved by multiple people. They also offer a multisig vault, which is basically an even more involved and more secure vault, requiring multiple keys to unlock.
On February 16, 2018, Coinbase admitted that some customers were overcharged in error for credit and debit purchases of cryptocurrencies. The problem was initiated when banks and card issuers changed the merchant category code (MCC) for cryptocurrency purchases earlier this month. This meant that cryptocurrency payments would now be processed as "cash advances", meaning that banks and credit card issuers could begin charging customers cash-advance fees for cryptocurrency purchases. Any customers who purchased cryptocurrency on their exchange between January 22 and February 11, 2018 could have been affected. At first, Visa blamed Coinbase, telling the Financial Times on February 16 that it had "not made any systems changes that would result in the duplicate transactions cardholders are reporting." However, the latest statement from Visa and Worldpay on the Coinbase blog clarifies: "This issue was not caused by Coinbase."