Tough to say their value.
Unfortunately for a seller, Kawasaki sold gobs of '78 and '79 Invaders....they were very popular when introduced.
The later model years are a bit more rare, when all the manufacturer's cut back due to slow sales (low snow years, high interest rates).
I still see a lot of Invaders for sale in the various ads in Indiana and Michigan.
They typically range from $200 in fairly rough shape up to maybe $600-700 for one's that are decent and in running condition.
There were so many Invaders produced that they don't bring near the price of the LTD models, etc.
I see you're in Oregon.....I don't have a clue what the vintage sled market is like there.
You mentioned a '78 free air. Kawasaki only made a free air 340 Drifter model in 1980.
Is it a ' 76 or '77 SnoJet by Kawasaki?
If it is the 1980 free air Drifter, they bring a surprising amount of money because they are pretty rare.