The one factory many brand things is true.
The ones I know are all the same are Panasonic, JVC, Fuji, Maxell, Canon and TDK. I was told this by Panasonic who also buy them in and brand them.
Having said that, unless you are 100% that the tapes you are using are all out of the same factory, don't mix (as others have said).
I currently pay £2.29 each for them and buy 10 at a time from
www.pricebuster.org.uk but watch out as there shipping charges can be a bit high so either buy in bulk or they do have shops in Manchester, Harrow and London.