I've got a POSP 4X on my arsenal ak74 that was an incredible value. You should be able to find one within your budget. You will likely need to replace the mount though to make it attach securely to your saiga. Kalinka optics has them at good prices.
In my experience all the low end, cheap side rail mounts are junk that either break or flex too much to hold a zero.
In my experience, I've seen some more expensive Eastern block rail mounts literally break apart while mounting them to the gun. The UTG I linked has been installed/taken off of multiple guns more than 100 times and still holds a zero.
I have 2 POSP scopes. 1 in 4x on my 7.62x39. and 1 in 6x on my .308. Both are decent scopes for the price i payed. BUT are harder to mount correctly than the UTG i linked. AND both have very rough windage/elevation adjustments. Each click moves the reticle ALOT.
You can see both below:
These scopes are also located to the left of the receiver by a good amount. If you're left handed, avoid them. The UTG mounts almost directly above the center.
Here is a photo of the Red-dot on the UTG on my 5.45x39
Purchase whatever you want. I'm just giving you the wisdom of my personal experiences. The UTG has held up to thousands of rounds of .308, 7.62x39, 5.45x39, 20ga, and 12ga. (I only own ONE mount) and has been swapped between guns many times. It is a better quality than many Eastern Bloc railed side-mounts and it puts the optic almost dead center.
If I were buying another rail for an AK, i'd get the same UTG I have. It is the best bang for the buck.
~SN
One more thing. At $200 each for a 4x24 or 6x42 POSP, you'd still have $170 for a scope and red dot with the UTG. A decent 3-9x40 Nikon Prostaff will run you $100. Just saying.