Hi Rosie 
Yes I went on Moonpay to check for future reference if there were other ways to do it and I found out the same $30 deposit as well. This is because 1 SOL = ~$30-36
But if you buy USDC on something like FTX or Binance etc, You’d be able to buy SOL for only the amount you supplied in usdc
But good news is 1 SOL can pretty much last a looong time! Since transfers take up some 0.0002 or something!