This is because of procurement procedures that Saudis follow are heavily laden with kick-backs and commission. Even when Pakistan tried to sell equipment, e.g. Al-Khalid tanks, Saudis bought more expensive Abrams. The bigger a deal the more the chance of commission going into the pockets of Saudi Royals.
A recent example is allegations of Hundreds of Millions of Dollars commission to Prince Bandar from British BAE systems. Even the investigation of this matter was terminated by SFO (Serious Fraud Office) in the interest of National Security i.e. Benjamins.
Add to that the US influence under which KSA is forced to buy downgraded US armaments, even during those times when they had budget deficit (US loaned KSA to buy weapons from them)
In my personal opinion, due to the depth of Pak-KSA relationships if Saudis show an intention for defence collaboration, Pakistanis will be willing to co-operate.