The SAKA, made by Richard Mumford, was $125 (much of that is the cost of the Petzl BASIC that it uses for the ascender) and the Powerscender is made by Mike Powers and I sent him the CT foot ascender (about $70) used for it, and I believe the cost for the set was $55 but you would have to check with him. My memory isn't what it used to be. For the ascender to point toward the rear (your heel) on the spike setup, you need to send him a LEFT foot ascender for use on your RIGHT climber.
The SAKA, of course, can be used with this setup or any spikeless ropewalker system, as well. You only need one of those for both techniques.
How beneficial a ropewalker system is to you will depend on a number of things... your age, the trees you're climbing, etc. but for most folks, walking up a flight of stairs is easier than climbing a tree with spurs. A ropewalker setup is pretty much like walking up stairs. You're using your large leg muscles. You don't have to pull yourself up with your arms at all. You just need to stay in close to the rope, so you're really just using your arms for that.