Our bill for ZenDesk would be around $300/month - we have more than 3 agents. (we actually use SalesForce for helpdesk so we pay more than that)

For a business if it saves us a support case a week it is worth the $100 a month. Anything under $100/month is essentially the same price as soon as you reach any sort of scale/revenue. (I am sure this goes up as you grow as well)