There is a lens, a old Ektar that seems to have a misty appearance when the lens is viewed. Is this something which can be rectified at reasonable cost or should I avoid any lens with this problem?

It could just need cleaning. A mixture of household ammonia and hydrogen peroxide will often remove stubborn haze. It could just be an accumulation of atmospheric crud, which would clean easily.

What makes a lens go cloudy?
I thought that it was a change inside, not the surfaces which meet the outside air.
I assumed it was a change that occured in the cement which would need such major intervention it would not be economically viable.

The cement could be a possibility, I guess, though personally I have not run across this. Extreme cloudiness is probably a serious problem that can be very expensive to fix. Minor haze that is barely noticeable is usually just accumulated crud that is easily cleaned. It can get in between elements inside the groups, where you would not think it could. Lens assemblies are not air tight and they breath in and out with changes in temperature. When they breath in as they cool, in comes te atmosphere with all it's various nasties. If the lens has been damaged by moisture getting in, that's a whole 'nother problem. Only a close up and personal inspection, preferrably with a loupe, will give you the answer.

Just what kind of Ektar is this? If it's the common 127mm, it's easy enough to dissassemble it and clean off haze, once you get a rubber block cut to unscrew the trim ring in front. And that might not even be nessessary.
Haze on old lenses is caused by gereral crud and the oils in the shutter lube breaking down. If no fungus is involve it's usually easy to clean all the air/glass surfaces involved.
Top

It is a 8 1/2" f6.3 Ektar in a Ilex #3 shutter.
Did Ansel use one of these?