It's my understanding that KSA largely buys its military equipment from the US (certainly in regard to Army equipment and its air force). I'm pretty sure they're one of the countries approved for F-35 export, and they're known to be F-16 and F-15 customers. The F-35 is not cheap, the other two are relatively cheap (still in the millions).

I don't think anyone is expecting KSA to declare war against Russia, but they're currently at war w/ (part of) Yemen, and they're not exactly happy with Iran, either. The middle east is set to explode, even without US involvement.