I shoot portraits - and a lot of the advice on here is linked back to never shoot a portrait under 50mm....well guess what....I do.
I shoot Nikon but I have an 18-70mm lens, and both myself and DiddyDave on this forum, both use it in the studio as its a superb lens for the money. We even use it for some shots at weddings.
A lot has to be said for the 50mm mainly because its a fast lens f1.8 or f1.4 and I have used it for portraits, and didnt really like it, as you have to be spot on with the focussing - and sometimes you might want BOTH eyes sharp due to the shallow DOF when using it wide open.
Plus if you wanted to do close up head/shoulders and also full length, then you'd need to keep running closer/further away from your subject.
I have used the Tamron 90mm f2.8 for candids at weddings, and have to say, depending where you are you really do suffer.
If you are using a cropped sensor body, then you also need to take into consideration that a 28mm isnt really a 28mm, and therefore a 50mm isnt a 50mm....
We shoot under 50mm, we shoot at 18mm, and we shoot at 70mm.
May I also suggest that that OP has said that he wants a cheap portrait lens, he hasnt said what type of portraits that he'll be taking, where he'll be taking them.
If he was taking them in the studio, then I wouldnt recommend the 50mm due to him not needing it at its f1.8 [or 1.4] and hes getting started in portrait photography.
I am sure that the OP wont mind me mentioning that I have spoken to him on MSN this evening regarding this, and have advised that the money for a "new lens for portraits" is probably best spent on learning about taking portraits first, and trying with his "kit" lens - and then deciding whether or not he needs any additonal lens'/flash/lights etc.