The hull and subs on your average T3 is going to set you back around 500-700mil.
…and a HAC, HIC, Force Recon, Combat Recon, Logi and Fleet Command ship is going to set you back just over one billion. That means the T3 costs 0.5–0.7× what the T2 equivalent would do — not 4–5× as much. You're off by almost an order of magnitude. If you stupidly choose to ignore the versatility — the neat trick you actually pay for
— and thus disqualify yourself from making any kind of sensible comparison, it's still only maybe 2× for a single function, as Riot Girl points out…
If you want to add the 500M worth of vanity fittings that people put onto their T3s, then we'll have to do the same to the T2 cruisers to maintain a reliable point of comparison, at which point they will close in on 1.5–2bn.
That's the number you need to compare against. And we haven't even gotten to the ridiculously short training path and the vastly simplified logistics the much cheaper price of a T3 buys you.