Whether it's a good idea or not is up to you. If you really want to host a dedicated server, then you have no choice. If you really don't want to buy a new key, then you can't host one.
It's your call on this. And to be quite honest, the game isn't that expensive.
If both the price and the need of hosting are too high, then you could always wait for a sale. Normally when Blockland is on sale on Steam, it's also on sale everywhere else you can pick it up.