There are many things to consider when contemplating a shift from OEM ink to 3rd party inks other than price which seemed to be the OP main concern.
If you are in the business of making prints for others, think about the following--
1) Is there any data available about the life of the third part inks, vs the OEM.
2) Since different pigments are used, even though the 3rd party inks advertise "matching", there will most likely be a difference in metemerism of the prints. (same image printed by the OEM & 3rd party, will appear different under different lighting conditions) This is a concern when trying to match prints for a client of previously printed work.
3) Already mentioned, "bronzing" or differential surface appearance and gloss or semi-gloss photo papers.
4) For HP printers, the grays used have been specifically matched to produce a neutral color. Do the 3rd party inks do likewise? This is a major concern if you print B&W mode on HP printers. (no metemerism)
If you consider all these things, plus the hassle of filling your cartridges, the 3rd party inks had better be significantly less in price than OEM. The biggest advantage of OEM inks is consistency of results.
John Nollendorfs