I usually buy higher-quality tissues and TP at discount houses like Wal-Mart, where prices on that kind of thing are much better.
Good toilet paper costs about 1/2 cent per square and tissues cost a little under a cent each, if you buy larger quantities, so 2 squares of TP is roughly equivalent to a sheet of tissue in cost, just a bit higher, but better-quality TP is probably softer on the nose than ordinary tissue.
On the other hand, if you are really pinching pennies, low-quality TP is much cheaper and probably tilts the cost equation the other way, but pop-up tissues in a box are much more convenient and can easily be grabbed with one hand.