Hi there, I don't think the steel has split down the seat tube, between the stays. I've just had a torch shining down the tube, and I think I can safely say that it is not a crack between the seat stays, but a deformity caused by whever tried to remove the post that was at some point stuck in the tube. There is also evidence of the offending post a way down the tube (quite a way I should say).
I've not really had any formal quotes on the work, though the post I added about viability of repair seems to indicate that it would not be a big job. Not being a welder or engineer, I'm no expert, but I think that all this would need would be some heat to reshape the tube a touch, and a bit of weld to fill the horizontal slit as seen in the pics. If it was my size, I think I would have got it done, but it is too big for me sadly. I may have also taken the opportunity to have the whole from resprayed.
Did you follow the link to the thread I posted about the repair? It seems that these frames were renowned for this type of injury.
Ian