I will chip in to say a lot of these issues are not limited to cost or buying a new war server box. A lot of that just comes down to how Minecraft is not intended as a game to have 150+ players in one area fighting it out in a couple of chunks. (As nickrocky said) No one really does exactly what we do in terms of throwing everyone into one area and just going for it. To get into a little more detail on how it works I’ll give you a little example that’s only kinda accurate. In this example packets are considered resources that the server has to process.
Player 1 and 2 can see each other. (6 packets)
Player 1,2 and 3 can see each other (12 packets)
Player 1,2,3 and 4 can see each other (24 packets)
Player 1,2,3,4 and 5 can see each other (48 packets)
As you add more and more players there is just a ton of more information that has to be sent from clients to the server and vice versa. It doesn’t scale normally – it multiplies a ton as more and more players are added. It gets out of hand very quickly. There is a ton of ways you can improve this such as running certain versions of Minecraft, making optimized plugins, or even doing something as basic as purging the area of entities so there is less information to be thrown around.