to get more cubits u can buy off mana shop. Or ingame with voting/buying. And to spend so /cubits spend (amount) and it will give u a code to enter in the discount section on the manacube website shop.
On Islands, the rate is about 1m for 0.01, so its pretty expensive, but skyblock's rates are much lower, so if you are trying to get them for cheap, hop on skyblock.