I'd say that Kolos has a good chance of losing value a good deal more as long as he is available in New Blood packs. You _might_ speculate on that.
Then again, there _have_ been exceptions to that rule and you will want him later. Speak about no strategy at all in including a specific card...
The price is likely to rise slowly once he is not available in New Blood any more.
The price for him should raise significantly in the last few days of his New blood Reign, if you sell him then, you might get away will selling him for 5k more than he's worth now at least...
(reason for this, is people tend to buy out popular cards from the end days of new blood packs, with intent to resell them later, as they'll be worth more..)
But, i personally would keep Kolos, cards you need to fear, are like Jackie, Lyse Teria Cr, Striker, Bryan...
but Kolos makes for a great threat, even better than Lamar in his day.(Threat, not card overall worth)
if you sit and play Kolos at a later time, or wait for them to go, you might be able to convince them that you're gonna One hit them in round 2-4 instead of 1, and it adds a slight fear factor.
if people are slightly afraid, they might make a mistake.
'sides, what other options are you looking into?
Rolph? Nistarok? Xu52? Miss Twice Cr?
or breaking it down into 2 4 stars, instead a 3 and a 5?