If you visit the shops on RT you'll get a guide for the price, and yes, sellers have marked the price higher for higher Hacked K-2 - K-4. For example, Soul's shop sells
Hacked K-1 @ 103.36 PED,
K-2 @ 178.91 PED,
K-3 @ 276.73 PED and
K-4 @ 358.10 PED.
As for what they're actually worth....
They only spawn when people kill Steampunks, unlike Panther pets which are present in their own spawns. I assume this is one of the reasons for the (incremental) price tag, because judging the stats there is nothing particularly special from Hacked K-1 to K-4.
E.g. Hacked K-1
Passive effect = Pet and Taming
Strength 10%
Level 14 unlock
Energy 14
Hacked K-2, K-3 and K-4: Same stats as above, apart from metabolic rate which is 21, 22 and 23 respectively.