The Spire Fleet isn't invincible. Even with an armored limousine (AT) to drive them around at breakneck speeds, and with the OMD down, they often get wiped out on homeworlds, and make me need to refleet them.
5 cities = 12 shipyards = 52 FF + 12 DD + 6 CA + 3 BB + 1 DN = 52*(88,000) + 12*(200,000) + 6*(500,000) + 3*(1,000,000) + 1*(2,000,000) = 15mil M for the hulls.
Add on the cost of railguns (2x45k = 90k M per DD, 2x90k + 4x 45k = 360k M per CA, 2x 135k + 4x 90k + 2x 45k = 720k M per BB, 2x180k + 4x135k + 2x90k = 1mil M for the DN), and that's over 20 million metal to replace.
And I was mostly doing this before salvage, so even though I controlled 53 planets, I only had 21k metal income.
That's why I almost always turn around when I start losing Frigates -- because once you're losing Frigates, you're going to start losing stuff that's actually expensive.