Our large Minmatar POS are currently costing approx 700M/month to fuel, and citadels don't have ongoing fuel costs just to exist.
Astrahus - the medium citadels is what we will set up in our C1 and C2 wormholes. The C1 is for those that prefer the comfort of highsec nearby, and our C2 is for those that prefer to remain in wormholes with a little better PI.
The first citadels of are built in POS, with a significant time efficiency bonus. (Well maybe not the Palentine Keepstar - but that is a 'unique' proposition.)
Just because I have my eye on the 700M/month prize, does not stop me from keeping an eye out on citadel prices. When they first came out, the medium citadels were costing 9B, and are currently fairly rapidly. I expect that the mediums will settle around the 1.5 Bil mark for a while and I will be looking for a few more days of stabilised prices before I go on a spending spree.
From Fuzzworks - my current preferred market history site
I am also after a Fortizar for our C5. Yes ... the lure of capital escalations exists for some of us, and we will get around to it again one day ... honest (Nullsec is keeping me very busy, and Bob only knows what I am even training at the moment. All I know is something random I set up quite a while ago).
Unsurprisingly the Fortizar's are not heavily stocked. At roughly 10 times the cost price, and (I believe an even longer research time), they will be slower. Where there are a lot of mediums on market, there are precious few of these large citadels. I would like to spend 15B but I might be pushing it somewhat. Until now the Fortizars will be POS built, with the station based large citadels starting to come out very soon. I expect that will drop the prices somewhat once station based citadels come out ... maybe.
The Keepstars are not even available yet. 30 days for the first citadel off an unresearched blueprint, so the first off the factory lines that are POS based will be approximately 19 May, with the station based builds ones coming out roughly 28 May.