All weapons have their uses and benefits, just because you find the spear to be useless, probably because of the lack of it's ability to combo, doesn't mean that someone else can't use it well by utilizing it's range. I find the bow to be a worthy investment myself because despite the low damage against other enemies, being able to take out peasants without the risk of taking damage from the rest of the horde makes it less likely for you to be overwhelmed, plus it helps with taking out other archers and weakening enemies before they reach you.
I'm not going to say the pricing is perfectly balanced with each weapon's potential usefullness, but the reason it's so high is to make players spend their money more wisely and make sure that what they are buying is going to be useful enough until they can buy something else. Plus, even if the spear isn't always great on it's own, if it was low priced and everyone had it early on, then each wave would be a cakewalk as everything would be stabbed to death before it got close.