AT&T offer was $39 billion for the whole t-mobile with NO ASSUMPTION OF DEBT. If you read At&t/T-mobile announcement, it said "AT&T assumes no debt from T-Mobile USA or Deutsche Telekom and continues to have a strong balance sheet".
Currently, T-mobile is $20 billion company over $20 billion in DEBT. So, the total already exceeds AT&T offer of $39 billion!
T-mobile needs Sprint more than Sprint needs -Tmobile. So, there will be NO premium. However, both share holders can likely triple their money as there will be huge cost savings post merger.
One must not forget at the time AT&T made thier offer T-Mobile did not have PCS Spectrum or Subs.
So with that merger I be willing to say at todays share price one may see a bid about $30 and that would be a % of cash and stock and that would be about a 10% prem over the Dec.13 closeing price. Why that days closeing price ? Simple Math No company would wait 6mths. to place a merger bis and watch that companies stock climb during that period . A non binding bid today save a Major head acke 6mnths down the road