I'm conducting an airdrop to every eos holders(ie. about 163,000). I've done a requirement estimation and it has been found that around 20 mb ram is needed. 464.28 EOS is required to buy that much of ram as per current(22/06/2018) market price. Can I sell this much ram after my usage?
Technically depends on whatever smart contract you'll use for the tokens however I'll assume you're using the
You can sell this RAM only if you're able to unstake it, you cannot unstake RAM if it is currently being used, it is used in the multi-index containers which each row costs a portion of RAM.
eosio.token contract, there is a row for each holder of each symbol, therefore if you're sending to 163,000 STEVE tokens, you're going to need to pay for their 163,000 rows in that table + the CPU and bandwidth costs on top of that.
Once your RAM is committed to each of these rows, they will remain committed until a user has claimed it by transferring the token which then makes them the RAM payer or if the balance has reached a balance of 0.00 in which case the row is destroyed and RAM returned to whoever's RAM was used.