I find that the highest price of synth ascension is not pop growth - the extra assembly jobs you get from planet capitol buildings once you ascend take care of that - but pop migration.
Synths do not emigrate from overpopulated worlds of their own volition, and allowing resettlement will displease individualist factions (iirc some xenophile factions also want free migration). This shuts down mid-late boost to new colonies that organic migration provides, and threatens to undermine your older colonies with overcrowding and unemployment unless you manually suppress assembly jobs, or have a faction setup that permits manually exporting excess pops.
Having to spend engineering research for the special projects is the second highest price. There are so many techs in engineering!