Dear Apple, u have to see the whole picture, prices vs sectors population.
As u mentioned safari valley is well populated, that is why u can get a good location 7 marla plot in 40+ there.
Where as other sectors which are with possession are much less populated, that is why u can get a 10 marla plot there still in less than 40 at some places, which is a very reasonable price for such plots anywhere in isb/ pindi in good areas.
U r right other sectors will take time to get populated but with that its prices will also start increasing and get out of common people reach. If u have more budget, u can always go for populated sectors but than the margin in price further increase is also less there. The sectors which are with possession or near possession and less priced are good for medium to short term investment. There prices can start increasing anytime. With the infrastructure of bahria, there are always chances of jumps here like we had in safari valley one year ago.