Protect its airspace from whom? The United States or Israel? Forget about that. Even if Iran would spend billions on buying Western or Russian air fighters, the US (and to some extent Israel) would have no problem to eliminate Iran's Air Force in case of a confrontation. That would not only be a unnecessary loss of money, but also an extremely shortsighted and dumb military strategy. You see, unlike KSA, Iran faces threats of the sole world superpower. No amount of money available would give Iran a conventional momentum against the US.
That is why Iran shifted away from its conventional military policy towards a more asymmetrical policy. To some extent the Iran-Iraq War, which caused enormous economical and social damages to Iran, has played a role in this change of policy. What the Iran-Iraq War taught Iran was that the West would not only allow Iran to gain a conventional victory in the Middle East, but also that its costs do not outweigh its gains.
Furthermore, what Iran foresaw and KSA didn't, was that military confrontations, regionally and globally, at the end of the 20st centuries were increasingly having a asymmetrical character. Palestinian resistance, the First Lebanon War, etc. were all signs that conventional power only is not enough to gain military victories. Another country in the Middle East, Syria, did not make efforts to change her military character too, and that is one of the reasons why it has difficulties to defeat rebel groups at the moment. Their conventional capabilities have proven to be unable to defeat small, mobile and determined rebel forces. Therefor, it is no surprise that the most capable and successful pro-Assad military organizations have a asymmetrical character, which Hezbollah and the Abu Fadl al Abbas groups are example of.
Besides Syria, KSA has failed to build up its own asymmetrical capabilities too. They have failed, after two years of supporting rebel groups in Syria, to unite the opposition, and are dependent on foreign military equipment to supply rebels groups. Iran on the other hand has 100% self-sufficiency in producing low-intensity military equipment, and therefor is more independent in order protect its own interests. Iran, like KSA, has no problem to supply pro-Assad forces with indigenous-produced drones, MANPADs, rifles, munition, artillery, ATMs, etc. KSA on the other hand is dependent on countries like Croatia to supply its forces in Syria.
Besides equipment, Iran has proven to be capable of setting up professional and disciplined pro-Iranian militias too. Hezbollah, who has enjoyed Iranian supplied and produced weapons and Iranian military training, managed to defend itself against Israeli forces in 2006, has demonstrated such effectiveness, while the militias that KSA has supported are usual extremely undisciplined, unreliable, unpredictable and ineffective to some extent. If KSA would have invested in these asymmetrical capabilities, I have no doubt that Assad would have been defeated already. But Assad has managed to withstand rebel advancement in the summer of 2012, and even to stage counter offensives.
People overlook the fact that we are in a new military era right now. The days of huge tank battles, air wars and infantry clashes are long gone. It is this insight that Iran has managed to deter outside forces from attacking her assets, and subsequently this misjudgment that has led to the removal of Saddam and Qadaffi. Naturally, if it wasn't for Iran, Assad's name would have been on this list too. Lucky for him Iran, unlike USA, does not betray its friends in times of adversity.
For example, Iran never supplied Hezbollah with Iranian-made weapons but they give them Russian's and Chinese in order to divert as much attention as possible. 
... And we can't make ice-cream here in Arabia! It's damn hot.
