It would cost much less than a backhoe and not a lot more than a bush hog. To run one on a CK20 you'll need a set of rear remotes ($350) or a WRLong type FEL diverter ($550). You have the option of a KL120 pin to universal skid steer adapter (about $600...though I just happen to have a used one for Kl120/130 I'd sell

) or get the
grapple company to build you a mount that fits directly to the KL120/130 pins. Then there is the
grapple itself. You have several choices there. I got the Millonzi 48" light duty which I think is perfect for the CKs and costs somewhere about $800 or a bit more if they fabricate the KL120/130 specific pins. Wildkat seems to have introduced a 48" model that is even less money based on a TBN members purchase a month or two ago at $500 (I still can't believe that price...maybe it was part of a package deal). WRLong and ATI make very nice light duty versions too but they cannot be purchased directly by the consumer and therefore cost more than double what the Millonzi does.
If your land clearing is half and acre or less perhaps you don't want to invest in this sort of equipment but for more than half and acre, you will find the combo of
grapple, bush hog and BH to be remarkably efficient and worth every dime. There is no comparison to what you can do with a bucket with or without toothbar and what you can do with a
grapple.
An alternative that saves relatively little money but is nice to consider is to simply add a
grapple to your standard bucket. It will be about 1/2 as efficient as a true
grapple but many times more effective than a bucket/toothbar alone. You'd still need to add the hydraulics but you save on the adapter and the add a
grapple kits generally can be had for about $500-600 which is cheaper than the full
grapple.