I know that we will need to buy around 4k of ram for each new account. When the account gets created it uses around 3k to store keys and other internal information. I know that this account can't ever be deleted. Suppose that later this account become inactive. Is there any way to claim back this ram to use in another account ? In this way the problem of user ram needs would be mitigated. If not this amount of ram will be lost forever as it will stay allocated uselessly.
There are several ways to deal with "lost" RAM. This is all new and might change in the future. RAM cost is determined by free market forces using a bancor algorithm to keep it liquid. Currently, the short answer is 'Yes', the RAM taken up for account management is locked.
If RAM will be very expensive, it will incentivize ways to free this RAM.
RAM itself might not be so expensive as it's supply will increase gradually over the coming years and additional side chains will enable faster new pools of RAM to be added to the network. The enabler for (almost) seamless side chains is "Inter Blockchain Communication" and should be added to EOS this year, 2018.