Hi Olga,
First question, which focal lengths, would you like your new lens to cover? I understand that you want it as an addition to your 16-50, right? so something like a 70-200 or 70-300, not a travel-zoom?
I think the Tamron SP 70-300 Di USD is actually a very good lens for the money (at least for the money you have to pay for it here in Europe). It is of course no G in build-quality and operation, but it is not really inferior to the Sony 70-300 G in image quality...
An even cheaper option which also delivers good image quality is the Sigma 70-300 DG APO Macro, but that one is not very well built. I use mine a lot, especially because it is very light weight, but i had it break two times in about 2 years (both times the AF in the lens broke), Sigma replaced the lens both times though. AF is also slower than the Tamron.
Lots of people also like the Minolta 70-210 f4 ("Beercan"), which can be had for very little money nowadays, but i did not like it that much. Very strong CAs at f4 and wierd color shifts (green in one part of the image, magenta in another) in high contrast situations...
The Minolta 100-300 D APO and the "big beercan" (Minolta 75-300) are also supposed to be good, but i never had one of them.
The Minolta and Sony 80-200 and 70-200 G options are all very, very good, but also very expensive, i guess those are the ones you are referring to as luxury options...