PAO group 4 have better cold temperature performance
Group 3 are less expensive I believe.
I believe the cheaper synthetic oils are mostly Group 3, while the more expensive synthetics also contain Group 4 PAO and sometimes Group 5 esters.
It's not all the base oil though, it's also the add pack ( VIIs, ZDDP, FM, TBN, etc ) that add to the overall quality (and expense) of the finished product.