Due to testing by the SWTOR Devs, some players were able to do a free server transfer yesterday. Don’t grow to attached to it though, as it will properly never happen again:

Today , 08:33 AM | #36

Hey folks,

Just to clear up any confusion. Yes, server transfers did end up free for a period of a few hours last night. However, this isn’t a permanent change. There is also no need to panic as this isn’t a precursor to anything.

We had received reports that players were experiencing problems over the past few days with Character Transfers. We were doing some testing with the Character Transfer service to resolve this issue and the price going to zero was a side effect of that testing. Some very observant players were able to benefit from that testing . Apologies for any confusion!

-eric