Questions as most of you are all in the know.....
1. Are expanders temporary until desired size achieved, then have to get custom gels to fill them once removed? Or are they as robust as gels?
In the past the expanders where considered as really temporary. The material has improved very much. Longer lifetime are now possible, but you will never get 100% guarantee that they do not leak or rupture. A mistake could be eg. to stab the needle (eg. a 21G or smaller) into the base plate or the sides of the valve. This bends the needle. When you pull out the membrane might not close correctly again. This logic also tells, the less fill-up the better for the valve integrity - still don't overdo the amount at a session. Maybe be flexible on the fill-up. Don't say 150 cc's if it is possible to have 300 cc's.
2. What are the biggest size gels available?
Don't know. I'm only interested in expanders ;-)
3. Is it possible to get expanders what could be over filled say to 5000cc and if so wha would be the staring size(under filled)
Hm. Let's take Beshine's example. With her 2nd surgery she had Eurosilicone expanders 3,000 cc rated. She filled them to 4000cc +/- 300cc. As far as I remember she said that one was leaking. Anyway if they do not deflate it should be possible to overfill them to 5000 cc.
Let's take Keisha's example. I think she has also a 3000 cc implant overfilled to 6000 cc's. Can't remember which one it was.
4. Does anyone know a rough idea of cost of such expanders?
I was never in contact with any surgeon or manufacturer, so I have no prices.
Look at Keisha Evans boob fund. It might be outdated, but there she gave approx. "surgery" costs for her desired size. Let's asume it is a bit more costly in Europe, maybe +20%. But it is only an estimate.
Any info would be greatly appreciated 
thanks xxx
Welcome!