Unless you are using a mod, upping your capital ship numbers is very expensive indeed.  First, you have to buy the research, and then you must drop another 3000 credits.  The result is that caps end up costing around 4800 credits plus metal/crystal.

But for the record, don't use them.  Late game, they are not used for their SC numbers so much as they are for their SC support abilities.  For instance, Repair Cloud vastly improves the survivability of SC.  Sure, they might increase your SC numbers a bit, but they are not cost-effective for their SC numbers later in the game..  Capitals, aside from the first 30 minutes, are support ships.

In single player... I am sure carrier cap spam is effective and usefull... but in multiplayer... your either going to end up doing some mean shizz if you do it right (2 haylcons+scouts+lf= very, very hard to lose early game)... but building 4 or more carrier caps... I only have seen done sucessfully twice... and thats only cause they were already winning by a large margin (and had many frigs)

Now.... as far as numbers... high level capital carriers are just as/more effective than carrier cruisers as far as fleet supply, but monies... 2 carrier caps will beat out carrier cruisers (including research and lab costs)... but a third carrier is really pushing it... cause your exp is getting divided and all... this doesnt take into accout ablities or fighting strength... and while raw damage of carrier caps suck... the sova missle batteries do serious DPS... haylcon... is... very very good... but a sirinka with healing and aura... very pathetic early game without a supporting fleet.  scramble bombers is better... (early game!!) if your not fighting anyone with fighters (like... against a TEC marza)

Generally after 4 capital ships, the cost of buying the crew upgrades becomes exorbitant.  Definitely if you want to go "fast carriers", then the capital ship variety is the way to do it.  If you want a sheer mass of strike craft, however, you'll need cruisers.