well it all depends on your needs, do you want it to do anything other than walk around? do you want to be able to program/use other commands than the built in ones?
mech rc, which one did you mean? shadow stalker is $621
robonova-I is $900
i-sobot is only $100
have you considered getting a chassis for a biped and using the axon as a main board? if you got the i-sobot and ripped out its guts you would have a biped chassis and something like 17 servos ready to use for just $230!
these other ones are terribly overpriced! but it is up to you with what you are looking for in a bi-ped.