It appears, from what I could find, that Saudia Arabia's yearly spending on education is $57.9B, they spend $42.7B on health, with a total spending of $229.3. So they spend 25% of their budget on education and less then 2% on arms importation. Yes they could spend that money on something else, but please don't make it seem like they are neglecting healthcare or educational spending for arms.
Perhaps considering the total military expenditure will provide a better perspective - $80B for Saudi Arabia in 2014; 3rd highest in the world in $ terms, and highest in the world in terms of percent of GDP (10.7%).
Unlike gold, the treasure Saudi Arabia is sitting on top of is: in higher demand, more easily convertible into wealth and prosperity, and among it's resource class, the most easily accessible in the world. That, combined with the fact they live in a dangerous part of the world, leads me to believe it's a rational decision to try to protect their treasure.
Whether their armed forced are capable is, sadly, depending on your POV, another story. Like in most autocratic societies, a strong military is seen as a threat to the leadership.
http://www.us-sabc.org/custom/news/details.cfm?id=1645