Hi rssc, I'll give it a shot. I'm sure I'll be corrected if I get it wrong.
Sten - S/N on receiver (the tube) it is my understanding that a "Stenling" conversion is ok. I'm not sure that a full Sterling conversion can be done on a Sten receiver as BATF has had some problems with welding up the cocking slot and re-cutting for the Sterling.
Sten - S/N not on the receiver (the tube). Some folks claim that the original manufacturer can move the S/N. I don't know so I will not comment. If the S/N is not on the tube, the receiver would not be a candidate for a conversion (as it has no S/N on it). Of course you can try anything you want. Old saying, play stupid games win stupid prizes. (Note: Some C&R Sten's have the S/N on the magazine well. Nobody in the 40's contacted BATF as to the correct location for the S/N, just talking foolish here. Just that S/N's have been all over the place on these guns.)
Price difference? I'd not be concerned if the gun is going to be unaltered. If it is to be cut-up into something else, the S/N not on the receiver would be a non-starter for me.
BTW, one thing some folks forget is that the "Model" box on the form will never change. You can try but BATF is using what was on the initial registration. That being said, if you convert a Sten Mk2 into a Lanchester, MP-28, or Stenling your paperwork will still say Sten MK2.
Above is my humble opinion. YMMV,
Enjoy,
K2