In fairness, that's a pretty small wireless network so I can see the cost of Clearpass (even just guest) might look heavy relatively speaking.
I don't want to get too commercially controvertial, but on the face of it that $ price looks a bit high? I'm just basing that on looking at my UK RRP prices and a loose understanding of discounts.
I recommend you disucss it with your regional Aruba representative and explain the situation. Maybe ask them for a few different partner companies who can quote you? If you can use the virtual version (assuming you have a vm host) I'd recommend that for sure.
I wouldn't want to recommend an alternative as all I can think of are either terrible, or more expensive!
Good luck!