OpenBTS is still 3500 USD so not much different than a commercial product and far more temperamental ...
the octasic products seem indeed very interesting, thanks dor pointing it out. however, without aiming for larger quantities, I doubt it is worth the r&d effort.
for now the easiest way to build a real bts would be to take a SDR, OpenBTS and add an Abis/IP interface to OpenBTS.
Regards,
--
Sent from a mobile device, excuse my short response